Abstract: The present invention is predicated on applicants' discovery that an appropriately spaced and dimensioned internal gap cladding can substantially reduce short wavelength cladding mode loss in a fiber Bragg grating. A fiber Bragg grating is provided with a ring of closely spaced, longitudinally extending gap regions in the glass peripherally surrounding the core. The gaps are spaced apart by thin glass webs having a thickness less than a wavelength of the light being transmitted and are disposed peripherally about the core at a distance of 2-10 wavelengths from the core center. The thin webs limit the passage of the light between the gaps. The combination of webs and gaps acts as an internal thin cladding which supports fewer cladding modes than conventional glass cladding and, significantly, provides increased wavelength spacing between the Bragg resonance and the first cladding mode resonance.

Abstract: A fiber Raman amplifier is configured to use a co-propagating Raman pump source, which may be beneficial in a variety of system configurations (for example, in bidirectional communication systems). By carefully configuring the pump source characteristics, sufficient optical gain can be achieved in the co-propagating arrangement, the characteristics including: (1) using an optical pump power of at least 50 mW, (2) having a relatively large spectral bandwidth within the pump (to suppress SBS); and (3) a frequency difference between all longitudinal pump modes of each pump laser being separated by at least the walk-off frequency between the pump laser frequency and the signal frequency, and all intense longitudinal modes between different pump lasers being separated by at least the electrical bandwidth of the communication system.

Abstract: Resonances in cross-phase-modulation (CPM) impairment occur in wavelength-division-multiplexed (WDM) transmission systems, having multiple equal dispersion-length amplifier spans, when channels have an integral number of bit walk-throughs per amplifier span. The CPM resonances are reduced by mis-matching the amplifier span lengths (and/or dispersion in each span) so an integral number of bit walk-throughs do not occur in successive amplifier spans. The CPM resonances are reduced by adding different lengths of dispersion-compensating fiber to each span, using different modulation bit rates and/or clock phase delay for each channel, and using different wavelength-selective clock phase delays for each channel.

Abstract: A fiber Raman amplifier is configured to use a co-propagating Raman pump source, which may be beneficial in a variety of system configurations (for example, in bidirectional communication systems). By carefully configuring the pump source characteristics, sufficient optical gain can be achieved in the co-propagating arrangement, the characteristics including: (1) using an optical pump power of at least 50 mW, (2) having a relatively large spectral bandwidth within the pump (to suppress SBS); and (3) a frequency difference between all longitudinal pump modes of each pump laser being separated by at least the walk-off frequency between the pump laser frequency and the signal frequency, and all intense longitudinal modes between different pump lasers being separated by at least the electrical bandwidth of the communication system.

Abstract: In accordance with the invention, an optical fiber is provided with a metal coating of controlled variable thickness by the steps of disposing the fiber in position for receiving coating metal from a metal source, and depositing metal while moving a shadow mask between the fiber and the source to provide patterning of deposited metal. Advantageously, the mask is translated at a constant velocity perpendicular to the fiber. The method is particularly useful for the fabrication of adjustable Bragg gratings.

Abstract: In soliton transmission, third order dispersion of the transmission fibers tends to cause unacceptable variation in the filter strength parameter, .eta. over the wide wavelength bands required for massive wavelength division multiplexing (WDM). The effect of dispersion is substantially eliminated by varying the mirror reflectivities R(.lambda.) of etalon filters with wavelength, such that the strength (.eta.) parameter, in soliton units, essentially remains at the optimal value across the entire WDM transmission band.

Abstract: Embodiments of the invention include an optical fiber system and method for separating the different wavelengths of transmitted light transmitted therethrough and for monitoring the respective optical power in the separated spectral components. More specifically, embodiments of the invention scan or modify the physical parameters of in-fiber gratings that couple light between spatially different modes of light within a wavelength-division-multiplexed (WDM) optical fiber system, separate the spatial modes using a mode-discriminating device (MDD) and monitor or detect the separated spectral components using a conventional or other suitable detector. By scanning the in-fiber gratings, the peak wavelength of coupling between two dissimilar modes is modified, thus allowing control of the coupling within the fiber optic system. Scanning the grating is performed, e.g., by changing the temperature or modifying the physical dimensions of the grating.

Abstract: A dispersive optical waveguide tap comprises a blazed refractive index grating in the core of the waveguide, coupling means, focusing means and utilization means. The grating is selected such that guided mode light of predetermined wavelength will, in the absence of the coupling means, be directed into one or more cladding modes of the waveguide. The presence of the coupling means, in optical co-operation with the waveguide, changes the guiding conditions such that the cladding modes are substantially eliminated from a portion of the waveguide that includes the cladding, whereby the grating directs the guided mode light into one or more radiation modes. The blaze angle typically is .ltoreq.15.degree.. The focusing means serve to bring the radiation mode light substantially to a focus in at least one dimension, the focal point (or line) depending on the wavelength of the light.

Abstract: A Brag reflective Mach-Zehnder filter has arms that introduce a .pi. phase difference in transmission but not in reflection. In one embodiment a .pi. phase difference is introduced in one of the two arms between the Bragg grating and one of the couplers. In another embodiment, a phase difference of .pi./2 is introduced in one arm both before and after the grating and the location of the grating in the other arm is shifted. The transmission of the resulting filter is substantially independent of the degree of coupling at the input and the output. WDM systems employing the new filters are also disclosed.

Abstract: A dispersive optical waveguide tap comprises a blazed and chirped refractive index grating in the core of the waveguide, coupling means and utilization means. The grating is selected such that guided mode light of predetermined wavelength will, in the absence of the coupling means, be directed into one or more cladding modes of the waveguide. The presence of the coupling means in optical co-operation with the waveguide, changes the guiding conditions such that the cladding modes are substantially eliminated from a portion of the waveguide that includes the cladding, whereby the grating directs the guided mode light into one or more radiation modes. The blaze angle typically is .ltoreq.15.degree.. The chirp serves to bring the radiation mode light substantially to a focus in at least one dimension, the focal point (or line) depending on the wavelength of the light.

Abstract: A dispersive optical waveguide tap comprises a blazed and chirped refractive index grating in the core of the waveguide, coupling means and utilization means. The grating is selected such that guided mode light of predetermined wavelength will, in the absence of the coupling means, be directed into one or more cladding modes of the waveguide. The presence of the coupling means in optical co-operation with the waveguide, changes the guiding conditions such that the cladding modes are substantially eliminated from a portion of the waveguide that includes the cladding, whereby the grating directs the guided mode light into one or more radiation modes. The blaze angle typically is .ltoreq.15.degree.. The chirp serves to bring the radiation mode light substantially to a focus in at least one dimension, the focal point (or line) depending on the wavelength of the light.

Abstract: The conversion efficiency of a cascaded Raman laser (CRL) can be significantly improved if it comprises one or more of the below recited design features. The CRL comprises an intracavity section between an input section and an output section. The CRL is adapted for receiving pump radiation of wavelength .lambda..sub.p, and for emitting output radiation of wavelength .lambda..sub.n >.lambda..sub.p, and each of the input section and output section comprises fiber Bragg gratings of center wavelengths .lambda..sub.1, .lambda..sub.2 . . . .lambda..sub.n, where n.gtoreq.2 and .lambda..sub.1 <.lambda..sub.2 < . . . .lambda..sub.n-1. Among the efficiency-increasing features is ordering of the fiber Bragg gratings such that in the input section and the output section the gratings of center wavelengths .lambda..sub.1 . . . .lambda..sub.n and .lambda..sub.1 . . . .lambda..sub.

Abstract: In accordance with the invention, a tunable fiber grating comprises a fiber grating secured to a magnetostrictive body so that magnetostrictive strain will be transmitted to the grating. An electromagnet is disposed adjacent the magnetostrictive body for applying a magnetic field along the body. Control of the current applied to the electromagnet permits control of the strain transmitted to the fiber grating, and thus control of the grating spacing and reflection frequency. In a preferred embodiment the magnetostrictive body is cylinder bonded along the grating. In alternative arrangements, the magnetostrictive effect can be mechanically amplified. An add/drop multiplexer employing the tunable gratings is described.

Abstract: Applicants have discovered that the intermodal beat noise of a fiber multimode laser can be substantially reduced by providing the fiber with an output coupler of broadened bandwidth. In a preferred embodiment, a reduced-noise, high power light source comprises a cladding pumped fiber laser having a chirped output grating. Experiments show that increasing the output bandwidth from 0.254 to 0.577 nm reduces the relative intensity-to-noise ratio 10 dB in a Nd-doped fiber laser. Increasing the bandwidth from 0.2 nm to 0.3 nm in a Yb-doped laser similarly reduces the noise by 12 dB.

Abstract: In accordance with the invention, a tunable fiber grating comprises a fiber grating secured between a pair of magnets so that magnetic force (repulsive or attractive) applied to the magnets is transmitted to the grating. An electromagnet is disposed adjacent the magnets for applying the field to magnetize them. Control of the current applied to the electromagnet permits control of the force transmitted to the fiber grating and, thus, control of the grating strain, spacing and reflection frequency. In a preferred embodiment the electromagnet is actuated to produce magnetic pulses which control the remanent force between the two magnets, eliminating the need for continuous power. An add/drop multiplexer employing the tunable gratings is described.

Abstract: Reflective mode coupling refractive index gratings are disclosed. The gratings can couple light of wavelength .lambda..sub.i in a fundamental spatial mode of the waveguide (e.g., LP.sub.01 to a reflected higher order spatial mode (e.g., LP.sub.11), substantially without reflection of any light of wavelength .lambda..sub.j .noteq..lambda..sub.i in a spectral range .DELTA..lambda..ltoreq.0.01 .lambda..sub.i. The mode coupling gratings (MCGs) can find a variety of uses in optical waveguide systems. Exemplarily, an MCG can serve as a wavelength-dependent loss element with abrupt (e.g., .about.1 nm) spectral dependence. However, a chirped grating with or without strength modulation can yield an MCG having relatively wide spectral dependence, including variable loss over a relatively wide (e.g., .about.10 nm) spectral range. Both types of MCGs are advantageously used in, for instance, optical waveguide amplifiers.

Abstract: In accordance with the invention, a continuously chirped fiber Bragg grating is made by fabricating a continuously chirped phase mask and using the mask to write a Bragg grating on a parallel fiber. The chirped phase mask is made by exposing a photoresist-coated mask substrate to two interfering beams: one a collimated beam and the other a beam reflected from a continuously curved mirror. After etching, the resulting phase mask can be used to write a chirped fiber grating having a continuously varying grating period without physical modification of the fiber. The resulting fiber grating has a widened bandwidth and uniform dispersive delay characteristics useful for dispersion compensation in critical telecommunications applications.

Abstract: The optical waveguide system comprises a mode discriminating coupler and reflective mode coupling means. These components can be combined in a variety of ways to perform a variety of functions. Among them are drop multiplex devices, add multiplex devices, add/drop multiplex devices, and power combiners. The device combinations can have low loss as well as high reliability, the latter due to the robust structure of the devices.

Abstract: The disclosed Mach-Zehnder (MZ)-type devices are planar waveguide devices, with interferometer arms of essentially equal length, with a maximum spacing between the arms (e.g., between the waveguide core centers) selected to make possible simultaneous exposure of both arms to refractive index-altering radiation. Exemplarily the maximum spacing is in the range 20-100 .mu.m. The simultaneous exposure of both waveguides makes it possible to form gratings of essential equal strength, such that typically no individual trimming is required. The resulting devices (typically add-drop filters) are substantially less sensitive to environmental changes (e.g., temperature gradients, mechanical vibrations) than prior art fiber-based devices, and are advantageously used in, for instance, WDM optical communication systems.

Abstract: A method for reducing the inherent polarization shift caused by birefringence between the TE and TM modes of an optical signal propagating in an optical grating which has a plurality of waveguides includes the step of: irradiating the waveguides of the optical grating for different periods of time to induce a compensating polarization shift that substantially reduces the inherent polarization shift. If desired, a compensating polarization shift may be induced which not only reduces but also substantially eliminates the inherent polarization shift.