Leland Stanford Junior University

Robert C Youngquist

Herbert J Shaw

Janet L Brooks

Kenneth A Fesler

Cassius C Cutler

Robert H Wentworth

Wayne V Sorin

Gordon S Kino

Byoung Y Kim

Moshe Tur

William P Risk

Kjell Blotekjaer

A sensor array and method for remotely monitoring environmental effects on a selected sensor. In a sensor array comprising a plurality of interferometers, an optical signal of a selected configuration is provided so that portions of the optical signal are propagated through the interferometers. The optical signal configuration causes optical signal portions which have traveled the same signal path, except where separated on signal paths in a selected interferometer, to coherently mix when combined at an output of the selected interferometer. No other optical signal portions in the sensor array will coherently mix. The optical signal comprises plural components such as side bands which are configured to form a power spectrum defining an optical signal coherence function having a maximum which repeats periodically at a rate substantially corresponding to optical signal travel time difference in the selected interferometer. Also, the coherence function has minimums or zero values at times such that intervals between various minimums substantially correspond to optical signal travel time differences through non-selected interferometers, thus precluding coherent mixing of optical signal portions emerging from those non-selected interferometers. By modifying characteristics of the optical signal components, one may change the repetition period of the coherence function maximum and of selected coherence function minimums to produce coherent mixing of signal portions from another interferometer while precluding coherent mixing of signals from all other interferometers, including the previously selected interferometer. Accordingly, a given sensor may be selected from monitoring, to the exclusion of all others.

Sensor array and method of selective interferometric sensing by use of

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.

Birefringent fiber narrowband polarization coupler and method of

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.

Single mode fiber optic single sideband modulator and method of

A fiber optic saturable absorber for processing optical signals comprises an optical fiber from which a portion of the cladding is removed to form a facing surface. A light-absorbing substance having non-linear light-absorbing characteristics is applied to the facing surface such that a portion of the optical signal energy is transferred from the fiber to the substance where it is absorbed. The device selectively attenuates the optical signal and noise, and can be used to reduce pulse waveform distortion caused by pulse broadening and by amplification of system noise.

Single-mode fiber optic saturable absorber

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.

Fiber optic modal coupler, interferometer and method of coupling spatial

A fiber optic frequency shifter comprises an optical fiber having two propagation modes and an acoustic transducer for generating an acoustic wave. The transducer is positioned relative to the fiber to cause the wave fronts of the acoustic wave to acoustically contact the fiber at an angle of incidence which is less than 90.degree. greater than 0.degree.. Preferably, the wavelength of the acoustic wave is substantially equal to the beat length of the fiber times the sine of the angle of incidence.

Acousto-optic frequency shifter using optical fiber and method of

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. In another preferred embodiment, the sensors comprise a plurality of interferometers connected in series configuration by a common optical fiber, which provides multiplexed output signals from the sensors to another plurality of interferometers comprising receivers. Functional equivalents of the series configuration provide sensors and receivers comprising birefringent or two-mode fiber. The optical path length differences between each pair of sensor arms are selected to prevent interference between the multiplexed sensor output signals from the various sensors. The optical path lengths through the sensors and receivers are structured so that each receiver produces a phase difference signal relating to conditions affecting light transmission through a specific sensor. A phase and amplitude modulation technique is disclosed for providing heterodyned output signals from the distributed sensor system.

Distributed sensor and method using coherence multiplexing of

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. In another preferred embodiment, the sensors comprise a plurality of Mach-Zehnder interferometers connected in series configuration by a common optical fiber, which provides multiplexed output signals from the sensors to a plurality of Mach-Zehnder interferometers comprising receivers. The optical path length differences between the arms of each of the sensors are selected to prevent interference between the multiplexed sensor output signals from various sensors. The optical path lengths through the sensors and receivers are structured so that each receiver produces a phase difference signal relating to conditions affecting light transmission through a specific sensor. A phase and amplitude modulation technique is disclosed for providing heterodyned output signals from the distributed sensor system.

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Mountain View, CA, United States of America

Robert C Youngquist