Abstract: This invention relates to optical fiber systems where light is launched into and propagated in the fundamental mode of graded-index multimode fiber. Generally, it is not possible to launch light exclusively into the fundamental mode of graded-index multimode fiber by butt-coupling the multimode fiber to an arbitrary fiber carrying light from a source; several other modes may be launched. By inserting a length of a special singlemode fiber between an arbitrary fiber carrying light from a source and the graded-index multimode fiber, light can be launched exclusively into the fundamental mode of the graded-index multimode fiber. The special singlemode fiber has a modal pattern that matches the fundamental Gaussian modal pattern of the graded-index multimode fiber, which permits launching of the light into only the fundamental mode of the multimode fiber. Several applications for propagation in the fundamental mode of graded-index multimode fiber arises.

Abstract: This invention relates to optical fiber communication systems where data light is launched into a very small set of the lowest order propagation modes of multimode graded-index fibers, resulting in very high data rate transmission capability. We have determined, experimentally and theoretically, that light launched into a small set of a few lowest order propagation modes, or launched only into the fundamental propagation mode of multimode graded-index fiber, converts into and within a limited small set of lowest order modes due to severe bending perturbations that may occur in deployed multimode graded-index fiber transmission cable. Low modal time dispersion of the limited small set of lowest order modes is much less than when all modes of the multimode graded-index fiber are launched, yielding a much higher data rate transmission capability. Added advantages are: low transmission loss, low modal noise, and data security.

Abstract: An optical planar waveguide wavelength division demultiplexer for separating multiple wavelengths received in a wavelength division multiplexed communication system is disclosed, with each wavelength launched into a separate output waveguide channel. Wavelength separation is performed by wavelength-selective angular back-reflection from Bragg gratings, which are fabricated into the channel junctions of the planar germania-enriched silica waveguide by applying short wavelength light past a mask. A second Bragg reflection, reflecting the same wavelength as the first Bragg grating, is introduced into each output channel circuit, in order to attenuate residual backscattered light at non-desired wavelengths. The Bragg grating is extended beyond the channel junctions into the merged channel for greater reflectivity, if required, for the double Bragg grating geometry.

Abstract: An intrusion-alarmed optical fiber communication system, where light from two or more sources are launched with a planar channel waveguide launcher into two or more modes of a multimode graded-index transmission fiber, is disclosed. Input fibers containing the source light waves and the output fibers are in direct contact with the waveguide channels. Waveguide channels redistribute the light from input channels to output channels by rerouting, crossing, merging, or splitting channels. Waveguide launcher precisely launches data light into the fundamental mode and intrusion monitor light into high order modes of a multimode graded-index fiber. Fiber intrusion attenuates light in high order modes, but much less of the data in the fundamental mode, thereby forming a basis for the intrusion-alarmed system.

Abstract: An intrusion-alarmed optical communication system in which two separate modal groups are launched into a single graded index transmission fiber. The system includes two light sources and a lens to focus the light sources into the transmission fiber. A spatial filter is used to selectively limit the angles of incidence of the light on the fiber, and operates in conjunction with the lens to launch light from one source at appropriate angles and points of excitation to excite higher-order modes in the fiber, and to launch light from the other source at appropriate angles and points of excitation to excite lower-order modes in the fiber.

Abstract: An optical communication system in which at least two separate modal groups are launched into a single graded index transmission fiber. The system includes two light sources and a lens to focus the light sources into the transmission fiber. A spatial filter is used to selectively limit the angles of incidence of the light on the fiber, and operates in conjunction with the lens to launch light from one source at appropriate angles and points of excitation to excite higher-order modes in the fiber, and to launch light from the other source at appropriate angles and points of excitation to excite lower-order modes in the fiber.

Abstract: A method for alignment of optical fiber connector halves, including the steps of securing a connector lens in a lens holder having a planar end face, forming a reference plane parallel to or coplanar with the planar face of the lens holder, and then positioning an optical fiber such that, if the reference plane is reflective, light launched into the lens from the fiber will be maximally reflected back into the fiber. In one disclosed embodiment of the invention, the lens holder planar surface and an end surface of the lens are ground and polished together to form a common reference plane. Three alternative approaches are disclosed for detecting light reflected back into the fiber. The first approach detects light in the fiber core by means of a beam-splitting mirror. The second approach relies on the detection of light in the fiber cladding material, which is minimized when the fiber is properly aligned with the lens.

Abstract: 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 microbend 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.

Abstract: 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.

Abstract: 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 microbend 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.

Abstract: 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 microbend 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.

Abstract: The specification describes a novel pyroelectric detector in which one input detector electrode is mounted directly on one surface of a pyroelectric substrate and is used for absorbing chopped incoming infrared radiation. The output detector electrodes are capacitively coupled to the reverse substrate surface and provide an AC coupled output signal with a minimum of conductive heat transfer from the pyroelectric substrate.

Abstract: Accurate pointing of an infrared receiver or transmitter is provided by boresighting using a special beamsplitter and visible spectrum transmitting optics. The beamsplitter reflects the infrared signal and transmits visible light which permits sighting through the receiver field optics for accurate boresighting or pointing.

Abstract: Apparatus is disclosed for modulating the R(18) or the R(24) lines of the (00.degree.1 - 10.degree.0) vibrational band of a C.sup.13 O.sub.2.sup.16 laser using Stark-tuned absorption resonances in N.sup.14 H.sub.3. A dc electric field is applied to a Stark cell containing N.sup.14 H.sub.3 gas to tune the asQ(6,6) absorbing transition in N.sup.14 H.sub.3 so as to overlap the R(18) laser line or the asQ(2,2) transition so as to overlap the R(24) line. Modulating frequencies approaching 300 MHz can be employed, and a 100% modulation depth is achievable with audio modulating frequencies. Unsaturated modulation has been obtained using a Stark cell of 10 cm length and containing N.sup.14 H.sub.3 gas at a pressure of 0.45 Torr for a laser power density of 12 watts per cm.sup.2.