A new class of single mode fiber optic sensors with preliminary data indicating possible applications are described. Basically, the sensor consists of a fused tapered biconical directional coupler surrounded by a stress birefringent medium. Light is launched into one of the input fibers and travels to the fused coupler sensor. Some of the input light is coupled into one or more output fibers within a fused region known as the waist. Ordinarily two output fibers carry the original light intensity, any difference being attributed to a constant excess loss. The two output fibers are each terminated at a photodiode. One of the photodiodes is coupled to the inverting input of a differential operational amplifier. The remaining photodiode is coupled to the non-inverting input. Under static conditions the ratio between the output fibers is a constant. Variations in input light intensity may be taken care of by employing difference over sum processing. It is generally known that the coupling ratio can be very dependent on the index of refraction surrounding the waist region. Thus, a medium which exhibits stress birefringence and is used to intimately surround the waist will cause the coupling ratio to change due to applied stress. Stress in the region surrounding the waist may be applied externally by a force which is transmitted directly into the medium. In the case of an accelerometer, the force is supplied by an attached proof mass. A variety of other physical observables may be sensed through mechanical actuators such as diaphragms, levers, electrostrictive, magnetostrictive, and thermostrictive elements. Internal stress producing mechanisms may be designed by mixing various sensitizing elements into the stress birefringent medium. For example, polystryene or nylon might serve as a sensitizer for acoustic sensors. Experimentally, the sensitivity of a number of couplers was studied by immersion in calibrated index of refraction oils. Coupler fabrication methods were shown to influence the sensitivity of otherwise identical couplers. The sensors were shown to have excellent low frequency response all the way to the steady state. Absence of "one-over-frequency" type noise makes these sensors particularly desirable for low or subhertz detection.