Abstract: There is disclosed herein a fiber optic directional coupler for coupling light between two polarization modes. A highly birefringent fiber optic waveguide is positioned on a flat polished surface with either of the principal axes of birefringence oriented at an angle, preferably 45.degree., to the vertical. A ridged block is then pressed down on the fiber. The ridges of the block have longitudinal axes transverse to the longitudinal axis of the fiber. The width of the ridge faces is one-half beat length, and the spacing between the ridges in also one-half beat length. The stressed regions caused by the ridges cause coupling of light traveling in one polarization mode into the other polarization mode by abrupt shifting of the axes of birefringence at the boundaries of the stressed regions.

Abstract: A fiber optic frequency shifter comprising two waveguides having different indices of refraction. In some embodiments the waveguides are two modes of propagation in one fiber. Plural distributed coupling ridges, or electrodes mounted adjacent piezoelectric materials, are independently driven to apply sinusoidally varying forces to the fiber. In some embodiments, the phase relationship of the driving signals for the electrodes or ridges is such that a travelling acoustic wave is launched in the fiber. In other embodiments, regions of stress in the fiber are created by an acoustic wave coupled into the fiber from a transducer coupled to an acoustic medium surrounding the fiber. The input carrier light is shifted in frequency by the frequency of the acoustic wave.

Abstract: A distributed sensor system using pulsed optical signals optionally produced by a short coherence length source to provide a phase difference output signal representative of conditions affecting a selected sensor. In one preferred embodiment, an array of fiber-optic sensors are organized in a ladder configuration, with the sensors positioned in spaced relation and defining the rungs of the ladder. Light pulses transmitted through the sensors are multiplexed onto a return arm of the ladder. The multiplexed signals are received by an optical fiber compensating interferometer which coherently couples portions of adjacent multiplexed light signals to produce a phase difference signal representing conditions influencing selected sensors.

Abstract: A modal coupler, for coupling between first and second order modes of an optical fiber, comprises a single continuous strand of optical fiber, and a device for applying stress to the optical fiber at spaced intervals along the fiber. The stress deforms the fiber and abruptly changes the fiber geometry at the beginning and end of each stressed region. The change in fiber geometry causes coupling of light from the fundamental mode to the second order mode. The coupler, under certain conditions, exhibits polarization dependence, and thus, it may be utilized as a fiber optic polarizer. In addition, the device couples coherently, and may be used in interferometric systems.

Abstract: A technique and system for accurate determination of differential propagation delays in fiber-optic circuits. The method includes providing a sinusoidally modulated optical signal to each of two waveguides defining optical paths. The optical signals received from the optical paths are combined to form a reference output signal which has a null waveform whenever the propagation delay between the optical signals contains an odd number of half periods of the optical signal waveforms. The difference in the sinusoidal modulation frequency producing a first and second null or constant waveform in the reference signal is determined. This difference value between adjacent frequencies forming the null or constant waveforms comprises the inverse of the difference of signal propagation delay in the two optical paths. Accuracy is improved by measuring the sinusoidal modulation frequencies corresponding to first and second waveforms which are not formed by adjacent frequencies.

Abstract: A distributed sensor system including an optical source having a short coherence length for optionally continuously monitoring each sensor in the system. In one preferred embodiment, an array of fiber-optic sensors are organized in a ladder configuration, with the sensors positioned in spaced relation and defining the rungs of the ladder. Light transmitted through the sensors is multiplexed onto a return arm of the ladder, with sensor spacing being such that interference between light from different sensors is prevented. The multiplexed signals are received by an optical fiber receiver which couples the multiplexed light with an interfering optical reference signal to produce a phase difference signal representing conditions influencing selected sensors. Embodiments are disclosed for use of either pulsed or continuous wave light sources.

Abstract: A distributed sensor system including an optical source having a short coherence length for optionally continuously monitoring each sensor in the system. In one preferred embodiment, an array of fiber-optic sensors are organized in a ladder configuration, with the sensors positioned in spaced relation and defining the rungs of the ladder. Light transmitted through the sensors is multiplexed onto a return arm of the ladder, with sensor spacing being such that interference between light from different sensors is prevented. The multiplexed signals are received by an optical fiber receiver which couples the multiplexed light with an interfering optical reference signal to produce a phase difference signal representing conditions influencing selected sensors. Embodiments are disclosed for use of either pulsed or continuous wave light sources.

Abstract: A fiber optic frequency shifter comprising two waveguides having different indices of refraction. In some embodiments the waveguides are two modes of propagation in one fiber. Plural distributed coupling ridges, or electrodes mounted adjacent piezoelectric materials, are independently driven to apply sinusoidally varying forces to the fiber. In some embodiments, the phase relationship of the driving signals for the electrodes or ridges is such that a travelling acoustic wave is launched in the fiber. In other embodiments, regions of stress in the fiber are created by an acoustic wave coupled into the fiber from a transducer coupled to an acoustic medium surrounding the fiber. The input carrier light is shifted in frequency by the frequency of the acoustic wave.