Mode coupling using grating of ridges and furrows to curve the fibre along a zig-zag path are associated with an index matcher which surrounds the fibre. The average axis of the zig-zag fibre coincides that of the matcher. Light travels along lines all at the same angle to the axis of the matcher and pass through via a conical optical end surface thereof. The optical surface converts the said rays into a parallel beam for coupling with large aperture optical means such as a photo-cell or a LED.
An optical fiber line arranging guide groove capable of sensing optical signals is provided for detecting the status of optical signals in the optical fiber line. Through an optical detection circuit and a display element, the connection status of optical signals for the optical fiber line in the optical fiber line arranging guide groove is determined.
In manufacturing optical transmission filters having double-refraction elements, extreme care is usually necessary to provide the desired phase delay. To simplify the manufacturing process while still achieving accurate phase delay, a filter is provided with at least one double-refraction element comprising a single-mode optical filter mounted between polarizers. The double refraction of the optical fiber is sufficiently weak so that the .lambda. length within which light beams propagating with orthogonal polarization states in the fiber are mutually delayed by 2.pi., is at least 1 cm. In one embodiment, the optical fiber comprises alternating sections which produce linear double refraction with sections which produce elliptical double refraction.
A system for remote measurement of structural forces includes a plurality of microbend transducers mounted along the length of the structure for microbending an optical fiber in response to structural forces, such as stress acting upon an oil or gas pipeline or the like. An optical time domain reflectometer (OTDR) has a light source for launching a pulsed optical signal for passage through the fiber and a photodetector for sensing as a function of time the intensity of backscattered light reflected back through the fiber, wherein this sensed time function is correlated directly with discrete longitudinal positions along the length of the fiber and the structure. When one or more of the microband transducers is activated to induce a microbend in the fiber in response to localized forces acting upon the structure, a portion of the backscattered light is lost at the microbend. This attenuation in backscattered light intensity is sensed quantitatively and positionally identified by the photodetector. Specific preferred constructions for microbend transducers and system arrangements particularly adapted for detecting structural strain in an oil or gas pipeline are disclosed.
A transparent optical cable 5 including a flexible core 5a and a flexible but stiffer cladding 5b transmits light signals axially through the core. When a transparent contact plate 7 or roller 10 depresses and flatteningly distorts the cable, the light signals leak out through the core-cladding interface at the depressed portion and are transmitted through the plate or roller to a photoelectric transducer 8 or 12 mounted on the plate or proximate the roller on a movable object 1, which converts the light signals to electrical signals. Signal transmissions into the cable are also possible, and the input-output signal coupling depression may be made at any point along the length of the cable.
Optical radiation can be efficiently removed from, or injected into, single mode optical fiber at an intermediate point along the fiber, by causing quasi-resonant coupling of the guided mode LP.sub.01 to an appropriate tunneling leaky (TL) mode, e.g., LP.sub.11. Such coupling is caused by means of a "grating" in the fiber, with the grating being formed by impressing a periodic (or pseudo-periodic) modulation on the fiber, or by causing a periodic (or pseudo-periodic) variation of the refractive index of the fiber by means of the photoelastic or the photorefractive effect. The nominal grating spacing .LAMBDA.(z) is chosen such that .OMEGA..sub.o =(2.pi./.LAMBDA.)>.beta..sub.01 -2.pi.n.sub.cl /.lambda..sub.o, where .LAMBDA. is the average grating spacing, .beta..sub.01 is the propagation constant of the LP.sub.01 mode, n.sub.cl is the refractive index of the fiber cladding, and .lambda..sub.o is the wavelength of the radiation to be coupled from or into the fiber. Furthermore, .LAMBDA.(z) is to be chosen such that .OMEGA..sub.o .about..beta..sub.01 -.beta..sub.rs, where .beta. .sub.rs is the propagation constant of the selected TL mode. In order for the coupling to be quasi-resonant, it is necessary that .alpha..sub.rs, the attenuation constant of the selected TL mode, be relatively small, typically <1 dB/cm. By appropriately choosing .LAMBDA.(z) and/or the amplitude function of the grating, it is possible to increase the coupling efficiency above what is possible with a constant spacing, constant amplitude grating. Devices according to the invention can be advantageously used not only as radiation couplers but also as filters and, if they are of the photorefractive type, as amplitude modulators.
