Abstract: For systems which disperse individual wavelength components of a DWDM beam into an array of converging beams, the individual wavelength signals are modified for blocking, equalization or other purposes by reflective liquid crystal cells. Thus modulated or modified components are then recombined by the system into an output beam, as by reverse passage through the system. Controlled full extinction or linear attenuation may be introduced by converging asymmetrical beams of separate polarization components for each wavelength into superposed relation on zero twist nematic crystal cells which are voltage controlled so as to retard for extinction of greater than 40 dB or to transform the state of polarization to a selected angle for attenuation. Polarization sensitive elements in the return paths of the reflected beams then filter the rejected components.

Abstract: Systems and methods for modifying, switching, rearranging or otherwise controlling the individual wavelength components of DWDM optical signals are described, which employ compact refolding and reshaping of these dimensionally patterned beams within a confined volume. The wavelength components of the beam are diffractively dispersed with high diffraction efficiency, and then reversely converged to beam waists incident on different ones of an array of control elements such as liquid crystal cells, MEMs and other spatial light modulators, or fixed distributed patterns. With reflective control elements the wavelength components may be reversely refolded along reciprocal paths with rediffraction, to form a reconstituted and revised DWDM output signal. If the control elements transmit at least one of the wavelength components, a separate, adjacent three dimensional beam refolding path, with rediffraction, is used to feed recombined signals to a separate output.

Abstract: Systems devices and methods in accordance with the invention impart high strength index of refraction patterns to photosensitive optical devices, such as Bragg gratings written in optical fibers. A length of small diameter fiber retaining photosensitivity is fabricated by flame elongation of an optical fiber precursor having dopant containing cladding, using a diffuse, low velocity inverted flame that does not introduce water, OH or H2 into the fiber. By varying the flame velocity during each scan the fiber is diminished to a small, uniform diameter, waist region. Photosensitivity is preserved and enhanced by exposure of the prepared waist region to scanning actinic illumination within an in-diffusing environment of pressurizing hydrogen or deuterium, and controlling the exposure to optimize the photo-induced index change.

Abstract: An add/drop filter for optical wave energy incorporates a Bragg grating in a very narrow waist region defined by merged lengths of elongated optical fibers. Light is propagated into the waist region via adiabatically tapered fibers and is transformed from two longitudinally adjacent fibers into two orthogonal modes within the air-glass waveguide of the waist and reflected off the grating from one fiber into the other. The geometry of the waist region is such that the reflected drop wavelength is polarization independent, without lossy peaks in the wavelength band of interest. Additionally, back reflection are shifted out of the wavelength band of interest. High strength gratings are written by photosensitizing the waist region fibers by constantly in-diffusing pressurized hydrogen or deuterium. For narrow spectral bandwidth gratings, dimensional variations must be minimized or compensated, and the grating is apodized by both a.c. and d.c. variations in writing beams at a net constant power.

Abstract: For systems which disperse individual wavelength components of a DWDM beam into an array of converging beams, the individual wavelength signals are modified for blocking, equalization or other purposes by reflective liquid crystal cells. Thus modulated or modified components are then recombined by the system into an output beam, as by reverse passage through the system. Controlled full extinction or linear attenuation may be introduced by converging asymmetrical beams of separate polarization components for each wavelength into superposed relation on zero twist nematic crystal cells which are voltage controlled so as to retard for extinction of greater than 40 dB or to transform the state of polarization to a selected angle for attenuation. Polarization sensitive elements in the return paths of the reflected beams then filter the rejected components.

Abstract: Asymmetric grating assisted couplers present unique problems if dispersion is to be reduced to levels at which high data rate signals can be transmitted. To overcome these problems, the method of the present invention includes precisely controlling and monitoring the cross-sectional geometry of the coupler during stretching and fusing. By monitoring states of polarization in lowest order modes as the coupler is formed, coupler fusion can be terminated when optimum form birefringence minimizing PMD is achieved. Dispersion is further minimized by impressing a saturated index of refraction grating with substantially flat add/drop characteristics in the coupler, and introducing a controlled amount of twist.

Abstract: Systems and methods for modifying, switching, rearranging or otherwise controlling the individual wavelength components of DWDM optical signals are described, which employ compact refolding and reshaping of these dimensionally patterned beams within a confined volume. The wavelength components of the beam are diffractively dispersed with high diffraction efficiency, and then reversely converged to beam waists incident on different ones of an array of control elements such as liquid crystal cells, MEMs and other spatial light modulators, or fixed distributed patterns. With reflective control elements the wavelength components may be reversely refolded along reciprocal paths with rediffraction, to form a reconstituted and revised DWDM output signal. If the control elements transmit at least one of the wavelength components, a separate, adjacent three dimensional beam refolding path, with rediffraction, is used to feed recombined signals to a separate output.

Abstract: Advantageous methodologies are disclosed for embedding periodic patterns in optical waveguide elements such as optical fibers. Polarization independence in an elongated waist region of a coupler can be established by measuring polarization characteristics during fusion and elongation, and controlling the heating and stretching to impart a cross-sectional shape, such as a hybrid dumbbell-ellipsoid produces a polarization insensitive drop wavelength. Alternatively, or additionally, polarization dependence can be minimized by angular deformation of the elements along its light transmissive axis. In addition, an element of relatively low photosensitivity is held in an hydrogen or deuterium environment pressurized to about 1000 to 5000 psi. While the environment is pressurized, a scanning UV beam is transmitted through a photomask and impinges on the coupler waist. In writing the grating, the in-diffused gas is constantly replenished, enabling the grating to grow.

Abstract: Optical filter devices in accordance with the invention are based on tapered optical fibers having transversely distributed refractive index variations in a small diameter waist region where waves are propagated in combined cladding/air-guided modes. The grating has a periodicity selected for reflection of a selected center wavelength and the waist diameter and grating pattern split the wavelengths of the lossy cladding modes from the backreflected signals by more than 10 nm. Such wavelength selective optical fiber devices have a variety of applications. In one application, a tapered fiber grating with optical circulators is used to add or drop optical signals for communication via a common transmission path. In another application, the tapered fiber grating is used with grating assisted mode couplers and circulators to form an add/drop multiplexer. In another application, these components are used with optical switches to produce programmable add/drop filters and crossconnects.

Abstract: A fast acting, low insertion loss switch for use in an optical fiber communication system includes a small diameter waist region incorporating a Bragg grating which is precisely tuned by tensioning to be reflective at a selective wavelength band. By a small physical shift of the waist, as by lateral displacement of an attached small magnetic element with a magnetic field, the periodicity of the grating is varied so that the selected wavelength is passed through the waist with virtually no loss. The incremental motion required can be introduced by a variety of devices, selected in accordance with cost, size and response time requirements. Modules having a number of such switches in close juxtaposition offer distinct advantages for wavelength division multiplexed systems.

Abstract: An optical signal filter for providing a periodic transfer function in transmitting signals within a selected bandwidth, by which passbands are interleavered into groups of separate outputs. The filter employs the transmissivity characteristic of birefringent crystals in conjunction with splitting the input beam into orthogonal and separate components, while compensating for temperature variations by pairing crystals of different types. The transmissivity functions are independent of the polarization of the input beam, and are shaped to flatten transmissivity peaks by the use of cascaded stages of birefringent crystal pairs.

Abstract: Precisely tensioned optical fiber devices are held in a temperature compensated package, with a small diameter span of a fiber containing a Bragg grating of a needed periodicity being supported under tension at opposite ends between spaced apart end members whose thermal expansion characteristics differ from that of an underlying base. The direct points of attachment of the ends of the tensioned span are rotationally as well as axially movable, enabling fine tuning of periodicity and twisting to minimize polarization and dispersion effects. This configuration is also useful for unique methods of assembly of the components, and for use during writing the Bragg grating in the small diameter span.

Abstract: A wavelength selective optical fiber coupler having various applications in the field of optical communications is disclosed. The coupler is composed of dissimilar waveguides in close proximity. A light induced, permanent index of refraction grating is recorded in the coupler waist The grating filters and transfers energy within a particular range of wavelengths from a first waveguide to a second waveguide. Transversely asymmetric gratings provide an efficient means of energy transfer. The coupler can be used to combine or multiplex a plurality of lasers operating at slightly different wavelengths into a single fiber. Other embodiments such as a dispersion compensator and gain flattening filter are disclosed.

Abstract: An add/drop filter for optical wave energy incorporates a Bragg grating in a very narrow waist region defined by merged lengths of elongated optical fibers. Light is propagated into the waist region via adiabatically tapered fibers and is transformed from two longitudinally adjacent fibers into two orthogonal modes within the air-glass waveguide of the waist and reflected off the grating from one fiber into the other. The geometry of the waist region is such that the reflected drop wavelength is polarization independent, without lossy peaks in the wavelength band of interest. Additionally, back reflection are shifted out of the wavelength band of interest. High strength gratings are written by photosensitizing the waist region fibers by constantly in-diffusing pressurized hydrogen or deuterium. For narrow spectral bandwidth gratings, dimensional variations must be minimized or compensated, and the grating is apodized by both a.c. and d.c. variations in writing beams at a net constant power.

Abstract: Using the wavelength selectivity and low loss characteristics of optical couplers, in which index of refraction gratings in a non-evanescent waveguide merged region reflect only a selected wavelength band, signal switching and control systems of novel characteristics are provided for multi-wavelength optical communication systems. In a router, for example, selected wavelengths or wavelength bands on an add/drop line can be added to or dropped from a main fiber, by cross switching between wavelength selective loops disposed sequentially in the lines. In a collector box, as another example, selected wavelength bundles can be routed from one fiber onto multiple fibers. In a bandpass filter a selected wavelength band is transmitted with low loss while those wavelengths outside the selected wavelength band are rejected with very high loss.

Abstract: Advantageous methodologies are disclosed for embedding periodic patterns in optical waveguide elements such as optical fibers. Polarization independence in an elongated waist region of an add/drop coupler can be established by measuring polarization characteristics during fusion and elongation, and controlling the elongation to impart a cross-sectional shape, such as a hybrid dumbbell-ellipsoid. Polarization dependence can also be minimized by angular deformation of the elements along the light transmissive axis. To write a pattern, an element having potential photosensitivity is markedly photosensitized in a hydrogen or deuterium environment pressurized to about 1000 to 5000 psi and a scanning UV beam that is transmitted through a photo mask and impinges on the coupler waist as the in-diffused gas is constantly replenished.

Abstract: Wavelength selective devices and subsystems having various applications in the field of optical communications are disclosed. These devices and subsystems are composed of bi-directional grating assisted mode couplers. The high add/drop efficiency and low loss of this coupler enable low loss wavelength selective elements such as optical switches, amplifiers, routers, and sources to be fabricated. The grating assisted mode coupler can be wavelength tuned by modifying the optical properties of the coupler interaction region. A programmable, wavelength selective router composed of multiple grating assisted mode couplers is also disclosed.

Abstract: A wavelength selective optical fiber coupler having various applications in the field of optical communications is disclosed. The coupler is composed of dissimilar waveguides in close proximity. A light induced, permanent index of refraction grating is recorded in the coupler waist. The grating filters and transfers energy within a particular range of wavelengths from a first waveguide to a second waveguide. Transversely asymmetric gratings provide an efficient means of energy transfer. The coupler can be used to combine or multiplex a plurality of lasers operating at slightly different wavelengths into a single fiber. Other embodiments such as a dispersion compensator and gain flattening filter are disclosed.