Leland Stanford Junior University

Optoplan As

Kjell Blotekjaer

Herbert J Shaw

Ole Henrik Waagaard

Byoung Y Kim

Sverre Knudsen

Erlend Ronnekleiv

Moshe Tur

Janet L Brooks

Robert C Youngquist

Dag Thingbo

Robert H Wentworth

Sigurd Lovseth

Sverre Kundsen

Methods and apparatus reduce coherence of an optical signal that is used to interrogate optical interferometric sensors. The optical field phasor of the interrogation source is modulated in a controlled manner to produce a broadened optical source power spectrum at the output of the source unit. The output from the source unit is launched into an optical sensor network, comprising a multiple of optical pathways from its input to the detection unit, where pairs of optical pathways form sensor interferometers. A compensating interferometer with delay difference similar to the sensor delay difference may be arranged in a serially coupled manner with the optical sensor network, either before or after the network. The coherence modulation may be performed through direct modulation of the source or through external modulation of the light with piezoelectric ring modulator, a Lithium niobate phase or intensity modulator, or an acoustooptic modulator.

Active coherence reduction for interferometer interrogation

A fiber optic coupler comprises a multimode fiber bus, an input branch fiber, and an output branch fiber. Coupling between the fibers is accomplished by means of a reflector, disposed in the core of the multimode optical fiber. The reflector is quite small, such that only a small fraction of the light propagating in the multimode fiber is incident thereon. The incident light is reflected towards the output branch fiber and coupled thereto through a lens. Light propagating in the input branch fiber towards the multimode fiber is focused on the reflector by means of a lens. The reflector is oriented to reflect the focused light in a generally cone-shaped beam directed longitudinally down the axis of a multimode fiber. Preferably, the cone-shaped beam diverges so that substantially all the modes within the acceptance cone of the multimode fiber are excited. Such excitation of the modes is preferably equalized by selecting the numerical aperture of the multimode fiber such that only the portion of the reflected beam which has a relatively uniform intensity is within the acceptance cone of the fiber. The invention is particularly advantageous for local area networks and distributed sensor systems.

Optical fiber coupler

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

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Trondheim, Norway

Kjell Blotekjaer