Abstract: In an optical fiber communication system that operates at a wavelength .lambda..sub.o and comprises optical fiber having a cut-off wavelength .lambda..sub.c >.lambda..sub.o, a "grating", with parameters chosen such that the grating causes resonant coupling between a predetermined guided mode (e.g., LP.sub.11) and a tunneling-leaky mode, can be advantageously used to remove unwanted guided modes. Such removal can substantially improve the bandwidth of such a system.

Abstract: An optical fiber sensing system for detecting intrusion of optical fiber or optical fiber cables includes an interferometric arrangement. Two ports (28, 30) of a four port splitter (25) are connected to a source (26) of optical power such as a laser, for example, and to a detector (32). The other two ports (46, 48) are connected to ends of a length (50) of monitoring optical fiber. An input signal to the splitter is split with one subsignal being directed in one direction around the length of optical fiber which serves as a common path between the two ports. The other signal is caused to travel around the common path in an opposite direction. The split signals are recombined in the splitter and their phase difference measured as a detectable pattern by the detector. Should there be intrusion of the optical fiber or a cable containing the monitoring optical fiber, the pattern which is detected will change a significant amount.

Abstract: 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.

Abstract: Optical fiber splicing method and apparatus. The method comprises aligning the ends of the fibers to be spliced by a procedure comprising coupling radiation into one of the fibers, the transmitting fiber, stripping nonfundamental mode radiation from the other fiber, the receiving fiber, and detecting the stripped radiation in such a manner that a substantial fraction of the detected radiation is lowest order nonfundamental mode (LP.sub.11) radiation for at least some relative position of the fiber ends. By adjusting the relative position of the fiber ends to minimize the detected power, the fibers are aligned. The procedure not only permits accurate alignment but can also be used to determine the splice loss. Apparatus for the practice of the method is also disclosed.