Showing papers on "Fiber Bragg grating published in 1991"

Bragg grating formation and germanosilicate fiber photosensitivity

Abstract: The refractive index of germanosiicate and germano—aluminosilicate fiber can be increased by a few parts in iO toupwards of five parts in i04 by photobleaching the oxygen vacancy defect band of germania. Typically, the fiber core is exposed through the side of the cladding to UV radiation at a wavelength from 240—250 nm. Permanent phase gratings are"written" with a specified period by using a pair ofintersecting coherent beams. We have investigated the fiber photosensi-tivity by monitoring the grating reflectivity and wavelength spectrum during the exposure. The Bragg wavelength shifts asthe grating is "written" due to a small increase in the average refractive index of the core. From these measurements, onecan determine the growth and saturation characteristics of the photoinduced changes and study the effects of composition and high temperature hydrogen annealing. We have also measured the polarization sensitivity and thermal stability ofgratings and the transient and permanent changes in fiber absorption.

Standing-wave monomode erbium fiber laser

Abstract: An integrated standing-wave, single frequency, erbium fiber laser having a laser linewidth of less than 47 kHz is reported. The monolithic fiber laser incorporated two highly reflective intracore Bragg reflectors which provide both cavity feedback and adequate longitudinal mode discrimination. The narrow linewidth single-mode operation and monolithic construction may make this a valuable communications/sensor source. >

All fiber, low threshold, widely tunable single-frequency, erbium-doped fiber ring laser with a tandem fiber Fabry-Perot filter

Abstract: An all fiber, widely tunable, single‐frequency, erbium‐doped fiber ring laser was constructed with a threshold pump power as low as 10 mW. Tuning over more than 30 nm was obtained by applying 0 to 17 dc V to an intracavity fiber Fabry–Perot filter. Threshold pump power versus wavelength data showed low variation over the tuning range. Mode hopping suppression with a tandem fiber Fabry–Perot filter is proposed and demonstrated. Stable single‐frequency operation was demonstrated with side mode suppression higher than 35 dB.

Multiplexing fiber bragg grating sensors

Abstract: Bragg reflection gratings and out-coupling taps for sensors can be written holographically within the core of many commercial fibers available today. The gratings appear to be permanent and have been tested to temperatures in excess of 500°C. Quasi-distributed temperature, strain, pressure, chemical, and interferometric sensors can be made with the wavelength selective, reflection gratings, and taps. The fiber gratings, and the different types of sensors they can make, conveniently lend themselves to (wavelength-division multiplexing) WDM, (time-division multiplexing) TDM, and (frequency-division multiplexing) FDM types of multiplexing schemes. Instrumentation to detect the multiple sensors and measure their spectral shift for localized and quasi-distributed sensing is currently under development.

Method of creating an index grating in an optical fiber and a mode converter using the index grating

Abstract: This invention relates to a method of creating an index grating in an optical fiber comprising disposing a slit mask containing one or more slits over a side of an optical fiber, illuminating the fiber through the slit mask by substantially monochromatic ultraviolet light for a short interval, whereby an index grating line is created and stored in the core of the fiber.

Helical-grating two-mode fiber spatial-mode coupler

Abstract: The authors investigate a helical-grating geometry for coupling the spatial modes in a two-mode fiber. The feasibility of making fiber grating filters based on this geometry is investigated. By adding mode strippers at the input and output of a helical-grating spatial-mode coupler, an optical notch filter is demonstrated with 0.3-dB waveguide loss, 24-dB extinction, and 24-nm spectral width. The effect of small ellipticity in the fiber core is also investigated, and it is shown that polarization-insensitive coupling can be achieved in a fiber with a sufficiently circular core. >

Method for producing a tunable erbium fiber laser

Abstract: A laser cavity is formed from two mirrors and an erbium fiber gain medium. One mirror is a dichroic mirror and the other is formed by a grating. The grating period determines the wavelength of light that is reflected, and can be changed by physically stretching or constricting the substrate to which the grating is attached. In one embodiment, the laser is tuned by adjusting the dimensions of the grating using a heating element. In another embodiment, the dimensions of the grating are adjusted using a piezoelectric ceramic material to which the grating is mounted.

Method and apparatus for the non-invasive measurement of pressure inside pipes using a fiber optic interferometer sensor

Abstract: A method and apparatus for the non-invasive sensing of the pressure within a pipe (or other vessel) is disclosed. An optical source produces a first light beam. This first light beam is split between a first (reference) and a second (measurement) optical fiber. The second optical fiber is associated with the pipe such that circumferential displacements in the pipe, due to changes in internal pressure, result in corresponding displacements in the length of the second optical fiber. Length changes in the optical fibers result in variations in the phase of the light emerging therefrom. The phase difference between the light beams emitted from the first and second optical fibers is then determined and related to changes in the internal pressure of the pipe.

Dispersion-induced composite second-order distortion at 1.5 mu m

Abstract: Experimental and theoretical results have shown that the composite second-order (CSO) nonlinearity of 1.5- mu m AM analog laser links is inadvertently affected by the coupling of laser chirp with fiber dispersion in regular single-mode fiber. To counteract the dispersion effect, it is shown that dispersion-shifted fiber can be employed in place of the regular fiber. Other possible ways to reduce distortion are to use lasers with reduced chirp, presumably with a multiquantum-well laser structure, or lasers with well-controlled spatial hole burning. Short fiber spans ( >

Optical waveguide comprising Bragg grating coupling means

Abstract: Disclosed are planar waveguides comprising substantially polarization-independent Bragg gratings. A preferred embodiment of the invention comprises a Si body with a silica lower cladding layer thereon, and a phosphorus P-doped silica core on the lower cladding. Appropriate periodic recessed features are etched into the core, and phophorus P- and B-doped silica upper cladding is deposited over the core. The dopant concentrations are adjusted such that the refractive index difference between core and upper cladding is small (0.35-1.45×10-2), and such that the flow temperature of the upper cladding material is lower than that of the core material. In another preferred embodiment a thin layer of Si3 Nx (x˜4) is conformally deposited over the core after the grating etch, and the upper cladding material is deposited onto the Si3 Nx layer. Bragg devices according to the invention are advantageously used in Integrated Optical Circuits (IOCs), e.g. in IOCs used in WDM optical communication systems.

Optical fiber acoustic sensor

Abstract: A new class of hydrophones is currently being developed, which utilize optical beams and interface directly to fiber optic transmission and processing links. Some of these sensors employ an optical fiber itself as the transduction element and possess significant advantages and versatility. The details of these fiber sensors and their advantages are discussed.

A three-electrode distributed Bragg reflector laser with 22 nm wavelength tuning range

Abstract: A large wavelength tuning range is demonstrated for a three-electrode distributed Bragg reflector (DBR) laser by applying both forward and reverse bias to the passive Bragg section. In the reverse direction the tuning towards longer wavelengths is mainly attributed to electrical heating. By also using the phase tuning section, all wavelengths within a range of 22.1 nm can be reached with a side-mode suppression better than 25 dB and with an output power exceeding 4.4 mW. >

Strain sensing using a fiber-optic Bragg grating

Abstract: This paper reports on the development of a passive, fast demodulation system for use with in-fiber Bragg gratings used for strain or temperature measurements. This compact, potentially inexpensive self-referencing system permits absolute strain/temperature measurements over a wide dynamic range with fast temporal response by tracking the wavelength shifts of the narrow-band back-reflected Bragg spectrum. The wavelength, bandwidth and strain sensitivity of a Bragg sensor are discussed, and examples of both static and dynamic strain measurements are shown.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Frequency stabilization of a tunable erbium-doped fiber laser

Abstract: A single-frequency Er-doped fiber laser that is tunable from 1.52 to 1.58 μm has been constructed. The laser linewidth was determined to be less than 1.6 MHz FWHM by observing the spectrum of the beat between the fiber laser and a 1.523-μm He–Ne laser. The frequency of the fiber laser was locked to several absorption lines of acetylene near 1.53 μm. This research demonstrates the inherent stability of fiber lasers and their potential for use in a wavelength standard for optical communications.

Photorefractive effect in bulk chalcogenide glass and devices made therefrom

Abstract: The photorefractive effect results in a change in the refractive index of bulk chalcogenide glass when it is exposed to certain wavelengths of light, in particular, sub-band-gap light. The effect results in a variety of device structures including a fiber optic faceplate; an optical waveguide; a fiber tap; a lens and a Bragg grating. Methods for fabricating the various devices are described.

Reflected optical power fiber test system

Abstract: A system for testing an optical fiber used in communications including a light source connectable to provide light to one end of a fiber under test, an optical power detector connectable to receive light from the same end of the fiber under test, a reflector at the other end of the fiber under test, and monitoring means operatively connected to the light source and detector to make a baseline measurement of optical power reflected by the reflector and subsequent measurements of optical power reflected by the reflector and compare the subsequent measurements with the baseline measurement to determine if there has been a change in optical power reflected by the reflector, the baseline measurement and the subsequent measurements involving no more than a single pulse of light from the light source.

LED-induced distributed Bragg reflection microwave filter with fiber-optically controlled change of center frequency via photoconductivity gratings

Abstract: A light-induced distributed Bragg reflection band-reject microwave filter is reported in which the grating elements are optoelectronically generated through periodic-structure photoexcitation of a silicon coplanar waveguide. The center for frequency can be optically adjusted to 11 GHz and 22 GHz by means of a pattern-controlled fiber bundle Array fed from six CW-operated 50-mW, 840-nm light emitting diodes (LEDs). Experimental results are in good agreement with theoretical predictions. The principle of operation demonstrated also applies to millimeter-wave integrated circuits. >

Optical fiber tension monitoring technique

Abstract: As an optical fiber (12) is being drawn, air (14) is directed at a portion of the fiber as a succession of air pulses, the pulses having a frequency near the natural frequency of the fiber portion. The frequency of the air pulses is then varied over a range of frequencies that includes the natural frequency of the fiber portion. When the air pulse frequency equals the natural frequency of the fiber portion, a resonance occurs which greatly amplifies the amplitude of the vibration of the fiber portion. The large deflection of the fiber that occurs at resonance is easy to detect, and the air pulse frequency which causes such maximum deflection is taken as being equal to the resonant frequency and therefore to the natural frequency of the fiber portion. Changes of the detected resonant frequency can be interpreted in a straightforward manner as changes in optical fiber tension which, in turn, are used to make compensatory changes of the temperature of the furnace (10).

Optical fiber light-amplifier system

Abstract: An improved optical fiber light-amplifier includes an amplifier optical fiber with a center core through which signal light is propagated, and a pumping light generating device which applies pumping light obliquely to the optical fiber. Pumping light is repetitively reflected within the optical fiber from portions of the optical fiber other than the interface between the core and regions surrounding the core. While pumping light is propagated through the fiber, it is absorbed by and excites the center core to amplify signal light being propagated through the optical fiber.

Photoinduced refractive-index changes in two-mode, elliptical-core fibers: sensing applications

Abstract: Photoinduced refractive-index changes in two-mode, elliptical-core optical fibers are shown to affect the differential phase modulation between the LP(01) and the LP(11)(even) modes. This change in beat length is dependent on the amount of strain induced in the fiber while the grating is being formed. We present experimental results that agree with conventional coupled-mode theory and propose the use of such sensors for weighted and distributed applications.

Fiber laser and method of making same

Abstract: A fiber optic laser includes a fiber optic element, and a Bragg grating means in the fiber optic element and defined by a periodic variation in the index of refraction of the element to enhance the narrow band response of the laser and/or increase the loss of unwanted frequencies. A method of making the fiber laser is also disclosed.

Mode-locked erbium fibre laser with wavelength selection by means of fibre Bragg grating reflector

Abstract: Mode-locked operation of a diode-pumped erbium doped fibre laser is described in which an optically written fibre Bragg reflection grating is used to define the operating wavelength. Pulses of 35 ps duration have been generated at a repetition rate of 5 GHz with mean (peak) output powers of 5mW(29mW).

Multiperiod-grating surface-emitting lasers

Abstract: Surface-emitting distributed feedback (DFB) lasers are disclosed with hybrid gratings A first-order grating is provided at one or both ends of the active region of the laser for retroreflection of light back into the active region, and a second-order or nonresonant grating is provided at the opposite end for coupling light out perpendicular to the surface of the laser or in some other selected direction The gratings may be curved to focus light retroreflected into the active region and to focus light coupled out to a point P When so focused to a point, the DFB laser may be part of a monolithic read head for a laser recorded disk, or an optical coupler into an optical fiber

Intrinsic fiber-optic sensors

Abstract: The basic theory of optical sensors is explained. State-of-the-art fiber-optic sensors are examined. There are two types of fiber-optic sensors: extrinsic and intrinsic. With extrinsic sensors, light leaves the fiber and is blocked or reflected before going back into the fiber-optic system. Extrinsic sensors are, in general, analogous to photoelectric controls with the potential problems of being affected by dirt, dust, vibration, and alignment. Intrinsic sensors avoid these problems by changing the light while it is still inside the fiber. Five types of intrinsic sensors are described. The advantages and disadvantages of intrinsic fiber-optic sensors are given. Present and future applications of intrinsic fiber-optic sensors are described. >

Spectral profile and multiplexing of Bragg gratings in photosensitive fiber.

Abstract: We describe a technique of optically writing several gratings in a photosensitive germanosilicate fiber. Each grating is written using the same single-frequency argon-ion laser source, but by varying the strain applied to the fiber during writing the resonant frequency of the gratings can be controlled. Frequency differences between gratings of 182 GHz have been demonstrated. The spectra of the gratings formed in the fiber were measured by periodically stretching the fiber while illuminating it with a single-frequency laser. This permits a frequency-resolved real-time analysis of the passband. The grating properties are also shown to be highly dependent on the polarization of the writing beam.

Method of forming optical fibre gratings

Abstract: An optical fibre (2) is fixed between a clamp (6) and a piezo-electric translation stage (10). An argon ion laser (12) emitting at 514.5 nm is used to write Bragg gratings in the fibre (2). A different longitudinal stress is applied to the fibre (2) before optical-writing each Bragg grating. The fibre (2) when unstressed will have a Bragg grating of different peak reflectivity corresponding to the number of different applied stresses.

Temperature distribution analyzer using optical fiber

Abstract: A temperature distribution analyzer is disclosed. In the analyzer, a measuring optical fiber is provided so as to extend through a target region in order to analyze the temperature distribution of the region. A LD pumped solid state laser generates pulse light having high energy and supplies the pulse light to one of the terminal portions of an optical fiber. This causes a stimulated-Raman-scattering phenomenon in the optical fiber whereby Raman-scattered light is obtained from the other terminal portion of the excitation optical fiber. An optical filter selects light having a predetermined wavelength from the Raman-scattered light and outputs the selected light. An optical device introduces the output light of the optical filter to one of the terminal portions of the measuring optical fiber and receives Raman-backscattered light generated in the measuring optical fiber through the same terminal portion. A photodetector converts the Raman-backscattered light to an electrical signal. A computing device determines temperatures of portions of the measuring optical fiber based on the variation over time of the electrical signal.

Shifts in resonance wavelengths of Bragg gratings during writing or bleaching experiments by UV illumination within germanosilicate optical fibre

Abstract: The resonance wavelengths of Bragg gratings are shown to increase with irradiation time during their inscription if the core of a germanosilicate fibre is exposed to a 243 nm UV fringe pattern. Exposure of the gratings to a 243 nm UV monotonic illumination leads to a recovery in the transmittance of the filters. Whether this irradiation induces a shift, or not, in the Bragg wavelength of the filters depends on the history of the grating writing experiment.

Fiber optic grating-based weighted, two-mode fiber sensors

Abstract: Two-mode, elliptical-core optic fibers with a permanent photo-induced index change are used as sensors with sensitivity varying as a function of length. The optic fiber sensors act as vibrational-mode filters thereby performing initial signal processing of the sensor signal. The sensors are based on photo-induced refractive index changes. These refractive index changes affect the differential phase modulation between the LP01 and the LP11 even modes. The change in beat-length is dependent on the amount of strain induced in the fiber while the grating is being formed. The pattern is thus varied along the length of the fiber by straining the fiber in a specific fashion while the grating is being written. This changes the sensitivity, of the sensor along its length. By choosing an appropriate weighting function in the manufacture of the sensor, it is possible to implement vibrational-mode analysis, vibrational-mode filtering and other functions that are critical in control system applications.