An optical fiber sensor includes: a central core disposed at a center of an optical fiber; and an outer peripheral core that spirally surrounds the central core. The effective refractive index n.sub.e2 of the outer peripheral core is lower than the effective refractive index n.sub.e1 of the central core. A ratio between the effective refractive index n.sub.e2 and the effective refractive index n.sub.e1 matches a ratio between an optical path length of the central core and an optical path length of the outer peripheral core.

An optical fiber includes multiple optical cores configured in the fiber including a set of primary cores and an auxiliary core. An interferometric measurement system uses measurements from the multiple primary cores to predict a response from the auxiliary core. The predicted auxiliary core response is compared with the actual auxiliary core response to determine if they differ by more than a predetermined amount, in which case the measurements from the multiple primary cores may be deemed unreliable.

Example embodiments include an optical interrogation system with a sensing fiber having a single core, the single core having multiple light propagating modes. Interferometric apparatus probes the single core multimode sensing fiber over a range of predetermined wavelengths and detects measurement interferometric data associated with the multiple light propagating modes of the single core for each predetermined wavelength in the range. Data processing circuitry processes the measurement interferometric data associated with the multiple light propagating modes of the single core to determine one or more shape-sensing parameters of the sensing fiber from which the shape of the fiber in three dimensions can be determined.

Provided is a cladded single crystal sapphire optical device (e.g., a s sapphire optical fiber or wafer). In one embodiment, the innovation provides a method for forming a cladding in a single crystal sapphire optical device by reactor irradiation. The reactor irradiation creates ions external to the optical device that enter the optical device, displace atoms in the optical device, and are implanted in the optical device, thus modifying the index of refraction of the optical device near the surface of the optical device and creating a cladding in the sapphire optical device.

An optical fiber sensor includes a first single mode fiber, a second single mode fiber, and a multimode fiber positioned between, and coupled to, the first single mode fiber and the second single mode fiber. The multimode fiber includes a graded-index core with an outer diameter between about 35 m and about 45 m. A numerical aperture of the core is between about 0.15 and about 0.25. The multimode fiber includes a cladding with an outer diameter between about 70 m and about 90 m. A coupling strength of an LP.sub.01 mode of the first single mode fiber to each of an LP.sub.02 mode and an LP.sub.03 mode of the multimode fiber is at least about 0.25.

Example embodiments include an optical interrogation system with a sensing fiber having a single core, the single core having multiple light propagating modes. Interferometric apparatus probes the single core multimode sensing fiber over a range of predetermined wavelengths and detects measurement interferometric data associated with the multiple light propagating modes of the single core for each predetermined wavelength in the range. Data processing circuitry processes the measurement interferometric data associated with the multiple light propagating modes of the single core to determine one or more shape-sensing parameters of the sensing fiber from which the shape of the fiber in three dimensions can be determined.

An extended length of optical fiber having an offset core with an inscribed Bragg grating is used a distributed sensor in combination with an optical frequency domain reflectometer (OFDR) to enable measurement small-scale (e.g., sub-millimeter) contortions and forces as applied to the fiber. The offset core may be disposed in a spiral configuration around the central axis of the optical fiber to improve the spatial resolution of the measurement. A reference surface exhibit a predetermined texture (in the form of a series of corrugations, for example, that may be periodic or aperiodic, as long as known a priori) is disposed adjacent to a longitudinal portion of the sensor fiber. The application of a force to the combination of the plate and the fiber creates a local strain in the grating formed along the offset core of the fiber that results in a shift in the Bragg wavelength of the grating. Using ODFR measurement techniques, an analysis of the Bragg wavelength shift allows for a high resolution force measurement to be obtained.

Provided is a cladded single crystal sapphire optical fiber. In one embodiment, the innovation provides a method for forming a cladding in a single crystal sapphire optical fiber by reactor irradiation. The reactor irradiation creates ions external to the fiber that enter the fiber, displace atoms in the fiber, and are implanted in the fiber, thus modifying the index of refraction of the fiber near the surface of the fiber and creating a cladding in the sapphire fiber.

A fiber optic cable assembly includes an elongate housing, a signal fiber placed inside the housing and extending longitudinally, and a plurality of sensing fibers placed inside the housing and extending longitudinally. The plurality of sensing fibers is placed around the signal fiber. Each of the plurality of sensing fibers carries a respective laser signal of a distinct frequency. The signal fiber carries one or more evanescent coupling signals responsive to the laser signals in the plurality of sensing fibers.

A fiber optic cable assembly includes an elongate housing, a signal fiber placed inside the housing and extending longitudinally, and a plurality of sensing fibers placed inside the housing and extending longitudinally. The plurality of sensing fibers is placed around the signal fiber. Each of the plurality of sensing fibers carries a respective laser signal of a distinct frequency. The signal fiber carries one or more evanescent coupling signals responsive to the laser signals in the plurality of sensing fibers.