Abstract: A multi-pass optical filter using two identical filters provides narrower band filtering by double filtering an optical signal passing therethrough. Although passing a beam of light at an optical filter having a substantially thick optical thickness is known to have significant coupling losses due to angular misalignment of the output beam and the receiving optical fibre, it has been found that twice filtering in the conventional manner by serially coupling two same filters increases coupling losses. However, this invention provides a coupling arrangement wherein losses are significantly lessened by offsetting the axes of the lensed filters such that the beam exiting the first filter couples into the second filter as it would be reflected backward into the first filter.

Abstract: A fault detection circuit and method of detecting catastrophic faults for example from a total line outage due to a transmission waveguide being inadvertently cut, includes tapping a sample of the amplified transmission multichannel signal light for analysis. The tapped light is then fed to a periodic filter such as a Mach Zehnder interferometer or a Fabry Perot etalon having a free spectral range and phase at one output that coincides with the channel central wavelengths of the transmission system. Another output of the periodic filter provides an output response that is phase shifted by 180 degrees. Two output signals from the two output two output signals are then compared, in one embodiment by using a photodiode bridge. When one output of the periodic filter has a substantially different signal energy than the other, it is assumed that no faults occur. Conversely when the two outputs are substantially similar, it is assumed that a catastrophic fault has occurred.

Abstract: An optical connector ferrule is provided having an standard single mode optical fiber therein whose mode field diameter is approximately 10 .mu.m along its length. An end portion and the end face of the optical fiber are heated to thermally expanded the mode field diameter to approximately 20 .mu.m or greater for reducing power density at the end face. By providing an expanded mode field diameter at the end face of the fiber, the power density is lessened, thereby lessening the probability that any dirt or debris at the end face will become heated sufficiently to damage the optical fiber.

Abstract: An optical coupler for single mode optical signals having a tunable (variable) coupling ratio, and a method of fabricating the coupler. A pair of virtually identical optical fibers are fused together at a narrowed waist region, each fiber being formed of a core and cladding, each being tapered toward the waist in such a manner as to ensure adiabatic propagation of light in the structure. An optical signal carried by one fiber first passes through a decreasing taper region and then passes through an increasing taper region of one or the other fiber (or both). The decreasing taper rate of the input portion of one fiber is such that the optical signal radiates out of the core (where V=1 locally) and into the cladding, as it approaches the waist. The increasing taper rates of the output portions of the two fibers are such that a predetermined coupling ratio is obtained.

Abstract: The modal-insensitive coupler consists of a number N of optical multimode fiber lengths, each fiber length having a core of initial predetermined cross-sectional area. The fiber lengths are fused together in a coupling section such that the cores of the fiber lengths have a reduced cross-sectional area in the coupling section which supports a minimum number of modes of optical energy at a predetermined wavelength, and such that the fiber lengths have different core cross-sectional areas to produce preselected coupling ratios.

Abstract: The optical reflective filter is made from a photosensitive optical fiber having a cladding and a core including germanium. Refractive index perturbations are light induced in the fiber in the region of the guided light. The perturbations can be made to have a constant or a varying periodicity, or a series of constant periodicities. The filters are made from single mode or low order mode fibers having germanium doped silica or germania cores. The center frequency for reflective band or bands has a wavelength in the 400 nm to 550 nm range. The reflective band or bands in the filter can be shifted by stretching the filter.

Abstract: Disclosed are optical data bus configurations for interconnecting terminals so that any terminal can communicate with any other terminal. It is known to do this by connecting terminals serially using two trunk optical fibers and to connect terminals together in a star configuration using a single transmission star or reflection star. The serial network is limited by optical loss while the star network requires more optical fiber and is more costly. Disclosed is a serial bus which uses six fibers arranged in two groups of 3 each, the groups conducting light in mutually opposite directions. A transmitting means in each terminal is coupled to two of the fibers in each group and terminals adjacent any one terminal are connected to a different two fibers than the one terminal. Another arrangement disclosed uses hybrid reflection-transmission star couplers. Terminals can be connected to reflection ports of the hybrid couplers, while the couplers can be interconnected by the transmission ports thereof.

Abstract: The optoelectronic switch includes a photosensitive GaAs FET onto which an optical signal may be directed. The optical signal is derived from an RF modulated light source. A voltage circuit is connected to the FET to switch the FET "on" or "off" by placing a positive or zero voltage respectively on the drain. An isolation of over 70 dB is obtained in this FET switch.

Abstract: The low-loss reflection star coupler has a bundle of multimode optical fibers which is folded on itself and fused over a predetermined length to form a bundle loop. During the fusion process, the fibers are elongated into biconical tapers in the fused section. A hybrid transmission-reflection star includes one or more further fibers having only one biconical tapered section which is fused to the fused section of the fiber bundle.

Abstract: The low loss access coupler includes two multimode optic fibers, each having a biconical taper section. The biconical taper sections of the fibers are fused together to provide optical coupling between the fibers. The fused fibers may also be twisted around one another to enhance mode mixing. The access couplers may be produced by fusing two fibers together along a small length, then heating the fused length and pulling the fibers to form the biconical tapers; or by twisting a portion of each of the fibers around one another, applying a tensile force to the twisted portions of the fibers and heating a region of the twisted fibers to soften and fuse a predetermined length of twisted fibers. If the fibers already have biconical taper sections, the access coupler may be produced by twisting the fibers together along their taper sections and heating a region of the taper sections to fuse them together.

Abstract: A CW Brillouin ring laser utilizing a low loss optical fiber and a pair of partially reflective mirrors positioned to form a ring optical path with the fiber ends, produces a Brillouin shifted Stokes beam when a pump beam is directed into one end of the optical fiber. The Brillouin beam propagates around the ring in a direction opposite to the pump beam. The pump beam is coupled into the ring through one of the partially reflective mirrors and the laser output is the portion of the Brillouin beam passing through the other partially reflective mirror. In the Brillouin laser system, a second pump beam may be directed into the other end of the fiber to produce a second Brillouin beam propagating in a direction opposite to the first Brillouin beam, providing a laser with two distinct Brillouin beam outputs.

Abstract: An optical coupler for a multi-mode optical fiber transmission line consisting of a first length of optical fiber and a second length of optical fiber positioned along a common axis such that one end of the second fiber is spaced a predetermined distance l from one end of the first fiber. This allows a portion of the optical energy leaving the end of first fiber to enter the second fiber. The coupler further includes a reflective surface facing the first fiber to reflect the remaining portion of the optical energy out of the coupler. The percentage of the energy reflected out of the coupler is determined by the distance l and may be varied by moving the second fiber with respect to the first. The material between the ends of the fibers has a refractive index substantially identical to the refractive index of the fiber cores to provide for greater efficiency.