Showing papers on "Single-mode optical fiber published in 1990"

Fibre optic single mode rotary joint

Abstract: The invention is a rotary joint for singlemode optical fibers, having a fixed and a rotating part to permit the transmission of optical signals across a rotational interface (such as a winch or turret) with minimal insertion loss and, in particular, low reflections (good return loss). There is no need of conversion to electrical signals; the device is passive. It may be use an oil of refractive index matched to that of the glass fibers and to that of fiber tapers or lenses used to expand the beam emitted from one fiber and contract it for transmission into the other fiber. The device is bidirectional. By use foil, through precision techniques for building and mounting the optical and mechanical components, and by use of advanced bearings, both the insertion loss and unwanted reflections (return loss) can be minimized, thereby making it suitable for use with singlemode fiber. Insertion loss can be further reduced in conjunction with index-matching fluid by using optical elements (lenses, tapers, fibers) having angled or curved facets rather than perpendicular facets. Oil filling has the further advantage of pressure compensation allowing the device to operate at any ambient pressure. Lenses having curved surfaces can be accommodated by the use of fluid having a refractive index different from that of the lens material.

Ultrashort-pulse propagation in optical fibers

Abstract: A more exact model is suggested for the description of nonlinear light propagation in fibers. In addition to the previously discussed self-phase modulation, parametric, dispersion, self-steepening, and Raman self-scattering effects, this model also takes into account the Stokes losses associated with the material excitation, the dependence of nonlinear effects on the light frequency, and the frequency dependence of the fiber mode area. The self-steepening effect is taken into account more correctly in comparison with previous models. The effects influence considerably the femtosecond soliton propagation. The model is generalized for the case of various fiber dispersion properties along the fiber length. The possibility of obtaining high-quality pulses of less than 15-fsec duration by compression of fundamental solitons with approximately 100-fsec duration in fibers with slowly decreasing dispersion is shown.

Statistical treatment of the evolution of the principal states of polarization in single-mode fibers

Abstract: A simple relationship is found for the evolution of the principal states of polarization (PSPs) and their differential group delay in fiber links. A simple expression is found, using the relationship, for the probability of the differential group delay (DGD), considering the evolution of the PSPs as a Brownian motion. The theory has been verified experimentally on an optical cable composed of 12 single-mode, shifted-dispersion fibers 2.2-km long. The results show that the DGD grows as the square root of the length when the length of the fiber is far larger than the correlation length of the perturbation. The measured value of DGD can vary substantially in two fibers belonging to the same ensemble, and in the same fiber, considering two frequencies differing by more than 5 nm. >

Bi-domain two-mode single crystal fiber devices

Abstract: A mode converter comprises an a-axis LiNbO3 optical fiber exhibiting a ferroelectric bi-domain structure. The fiber is subject to an electrical field that induces a +π/2 phase retardation in one domain of the fiber and a -π/2 phase retardation in the other domain. A light signal launched in the fundamental mode of the fiber is converted into a light signal propagating in the second order mode. When the electrical field is selected so that the phase retardations are not multiples of π/2, the mode conversion is partial and the LiNbO3 fiber can operate as an optical switch or as an amplitude modulator. The mode converter can also be operated as a second harmonic generator. The fiber is heated to a phase matching temperature so that a signal launched in the fundamental mode of the fiber and at a frequency ω is converted to the second order mode at a frequency 2ω. The LiNbO3 fiber can also simultaneously operate as an optical switch and as a second harmonic generator. Other non-linear interactions are possible such as sum or difference frequency generation or parametric generation. The various embodiments of the present invention are reciprocal.

Statistical properties of Rayleigh backscattering in single-mode fibers

Abstract: An analytical model that takes into account the influence of the laser linewidth on Rayleigh backscattering is given. For an unmodulated source the power spectral density is found to be identical to the delayed self-homodyne spectrum of the laser. The backscattered signal was measured for different laser linewidths, and the results are compared with the theory. The effects of Rayleigh backscattering noise on fiber gyroscopes and bidirectional optical systems are discussed. >

The effect of four-wave mixing in fibers on optical frequency-division multiplexed systems

Abstract: The limiting effects of four-wave mixing on optical frequency-division multiplexing (OFDM) systems are described. The optical nonlinearity in a single-mode fiber imposes a fundamental limitation on the capacity of optical frequency-division multiplexed systems. In particular, four-wave mixing (FWM) crosstalk may severely degrade the system performance when the fiber input powers are large and/or the channel spacing is too small. Theoretical and experimental results of the effects of FWM in OFDM systems are presented. The theoretical results demonstrate the dependence of FWM on various system parameters. An analysis of FWM in both undirectional and bidirectional transmission systems is included. The receiver sensitivity degradation from FWM crosstalk is measured in a 16-channel coherent system. >

Transverse mode characteristics of vertical cavity surface-emitting lasers

Abstract: Transverse mode characteristics of vertical cavity surface‐emitting (VC‐SE) lasers are described. The mode structure is investigated as a function of the transverse dimension for proton‐implanted gain‐guided VC‐SE lasers. A comparison is made to an air‐post index‐guided structure. The lasing modes and the evolution of the modes with increasing drive current for the VC‐SE lasers are observed to be highly analogous to those of the edge‐emitting lasers. Broad‐area gain‐guided lasers lase in the fundamental TEM00 mode near threshold. At higher currents, high‐order modes are successively excited. A 5 μm square proton‐implanted gain‐guided VC‐SE laser emits a single mode. On the other hand, an air‐post index‐guided SE laser, due to the large index difference between the laser and the cladding, emits multiple transverse modes. Moreover, we show that the gain‐guided VC‐SE lasers exhibit better device characteristics than the air‐post index‐guided lasers.

Optical fiber coupler

Abstract: The optical fiber coupler of the present invention is characterized in that, for a defined section in which two or more component optical fibers are thermally fused and elongated thereby forming a fused-elongated region which constitutes the optical fiber coupler, that at least one of the component optical fibers is formed from a single mode optical fiber material employed within a parameter range in which the mode field diameter increases monotonically with decrease in the diameter of the optical fiber core. The optical fiber coupler is further characterized in that, when using optical fibers from which a portion of the coating material has been removed thereby exposing the cladding, and by aligning two or more of such component optical fibers side by side in a plane and mutually thermally fusing the exposed cladding of adjacent optical fibers, after which the fused region is drawn out, thereby forming a fused-elongated region which constitutes the optical fiber coupler, for such an optical fiber coupler, optical fiber material is used in which the softening temperature of the core is higher than that of the cladding, and further, for the drawing out of the mutually fused sections, the tension employed is such that by virtue of remaining stress, the refractive index of the core is reduced.

Effect of gain and index nonlinearities on single-mode dynamics in semiconductor lasers

Abstract: The finite intraband relaxation time in semiconductor lasers leads to gain saturation at high laser powers. The nonperturbative solution of the single-mode density-matrix equations shows that both the optical gain and the refractive index become intensity dependent as a result of intraband relaxation dynamics. Gain and index nonlinearities are included in the rate equations, and how the modulation response and noise characteristics of semiconductor lasers are affected by such nonlinearities is studied. The intensity dependence of the frequency and the damping rate of relaxation oscillations leads to a fundamental limit imposed on the small-signal modulation bandwidth; the analysis provides an expression for the ultimate modulation bandwidth in terms of the material parameters. >

Efficient power coupling from a 980-nm, broad-area laser to a single-mode fiber using a wedge-shaped fiber endface

Abstract: A theoretical and experimental study of an efficient technique for coupling a broad-area laser to a single-mode fiber without the use of bulk optical components was carried out. The technique uses a wedge-shaped fiber endface to approximate a cylindrical lens action to correct for the phase mismatch between the curved laser beam wavefront and the planar fiber beam. A theoretical formula for the coupling efficiency in the absence of both angular and transverse misalignments is derived. By optimizing the wedge angle and the axial separation between the laser facet and the fiber endface, a maximum coupling efficiency of 47%, compared to 20% for the square endface, achieved, resulting in 15.2 mW of power in the single-mode fiber. The fabrication process consists of a simple polishing procedure requiring a wedge-shaped polishing tool and is highly reproducible. >

Bending effects in biconically tapered single-mode fibers

Abstract: Biconically tapered single-mode fibers were fabricated, and their characteristics were studied experimentally. The optical throughput was measured as the fiber was being pulled to produce the required radial profile. The tapered single-mode fiber was bent in a simple fixture, and the optical throughput was measured as a function of the bending angle. It is seen that the bending resulted in very strong oscillations of the optical power as the bend angle was varied. At the point when the light in the core was minimal, the cladding region was bright, indicating that the light has moved to the cladding. The propagation characteristics of the tapered single-mode fiber subjected to bending are analyzed using a stepwise approximation. A simple sensor based on this principle is discussed. >

Mode competition and control in higher-power gyrotron oscillators

Abstract: One of the most important problems in the design of high-power millimeter-wave sources such as gyrotron oscillators is insuring that the device operates in the desired mode. For high-power and short-wavelength devices the effective mode density is high, in that the current is above threshold for many modes. One then is led to ask whether operation in a single mode is possible and what steps must be taken to maximize the electronic efficiency of the device while ensuring single-mode operation. The answer to the first question has been determined to be yes. Provided that certain conditions are met, single-mode operation is stable. The present results emphasize time-dependent multimode simulations showing how these stable states can be accessed. In particular, the accessibility to the stable single mode with maximum efficiency is studied. Regions of parameter space for which stable single-mode operation is possible are plotted for an annular beam for a closed-cavity gyrotron operating at a high-order whispering-gallery mode (TE/sub 80.4/). These results also apply to the quasioptical gyrotron with a pencil electron beam. >

Fiber optic gyroscope

Abstract: In a fiber optic gyroscope comprising a light source, a fiber optic coupler, an optical integrated circuit having a function of a polarizer and a branching optical waveguide, and a fiber optic coil, and detecting an angular velocity applied to the fiber optic coil about the axis thereof, a polarization maintaining optical fiber having its length L is connected to an input/output end of the optical integrated circuit nearer the light source in an optical system of optical fiber from the light source to the optical integrated circuit, the polarization maintaining optical fiber of its length L resulting in a difference in group delay time between the orthogonal two polarization modes and the difference at least exceeding a coherence length of light from the light source. An optical system of optical fiber from the light source to the optical integrated circuit except for the polarization maintaining optical fiber of its length L is formed by a single mode optical fiber, and the fiber optic coupler is formed by a single mode optical fiber.

Thermal effects of Brillouin gain spectra in single-mode fibers

Abstract: Brillouin gain spectra in two 250-m-long single-mode fibers with GeO/sub 2/-doped core/pure-silica cladding (fiber A) and pure-silica core/F-doped cladding (fiber B) were measured at temperatures ranging from -40 to +60 degrees C at a wavelength of 1.32 mu m. The temperature coefficients of Brillouin frequency shift were found to be 1.17 and 1.33 MHz/ degrees C for fibers A and B, respectively. Temperature coefficients of Brillouin gain bandwidth were found to be -0.12 and -0.10 MHz/ degrees C. These measurements provide useful information for applications of stimulated Brillouin scattering. >

Analysis of fiber directional couplers and coupler half-blocks using a new simple model for single-mode fibers

Abstract: A model for a step-index fiber in the form of an equivalent slab guiding structure is presented. This model simplifies the analysis of fiber directional couplers and coupler half-blocks with dielectric and metallic superstrates. Simple empirical formulas to obtain the model parameters are also given. The use of this model is illustrated with some numerical examples. >

Interferometric stain measurement by arbitrarily configured surface-mounted, optical fibers

Abstract: The phase change of light traveling in a single-mode, arbitrarily configured optical fiber when the fiber is subjected to an arbitrary strain field is described by an integral equation. Assumptions concerning strain transfer from the strained body to a surface mounted optical-fiber sensor are experimentally verified. The general mathematical description is applied to two interferometric optical-fiber strain sensor geometries and is verified by experiment. The lack of precise data about the mechanical and optical material properties of the fiber is circumvented through calibration. Extensions of this model to both embedded fibers and nonlinear fiber response are discussed. >

A 970 nm strained‐layer InGaAs/GaAlAs quantum well laser for pumping an erbium‐doped optical fiber amplifier

Abstract: We report the performance of a 970 nm strained‐layer InGaAs/GaAlAs quantum well laser and its application for pumping Er‐doped optical fiber amplifiers. The laser was grown by molecular beam epitaxy and has three In0.2Ga0.8As/GaAs quantum wells. For a 5‐μm‐wide and 400‐μm‐long ridge‐waveguide laser, a cw threshold current of 20 mA and an external quantum efficiency of 0.28 mW/mA per facet were obtained. Maximum output power exceeds 32 mW/facet. With antireflection coating, even higher external quantum efficiency (0.40 mW/mA) was achieved, and more than 20 mW of power was coupled into a single mode fiber. Preliminary experiments of pumping the Er‐doped fiber amplifier gave 15 dB of gain at 1.555 μm for a pump power of 14 mW into the Er fiber.

Optical fiber sensor with localized sensing regions

Abstract: A coupling mechanism for a non-identical dual core optical fiber. The fiber is sensitized along selected portions of its length by coating the fiber in a pattern of bands of specified spacing with material that is sensitive to the presence of a predetermined parameter or environmental field. By virtue of inequality of propagation properties and mismatch of optical phase of the fields of the two cores, evanescent wave coupling is largely or wholly suppressed except in those portions of length coated in the specified spatially periodic manner. Optical power launched into one of the two cores remains propagating within that core alone over an extended length of the fiber, except for that period of time when the predetermined parameter or environmental field is sensed. The specific spatial periodicity of the pattern of coating bands on the outside of the fiber acts in concert with the nonidentical propagation parameters of the two cores to effect intercore coupling of optical energy. This coupling, representing sensitivity to any environmental field which causes strain in the coating, is effective only on those portions of the dual core fiber length which bear a specific coating pattern.

Integrated laser-amplifier with steerable beam

Abstract: A monolithic integrated master oscillator power amplifier (MOPA) device including a single mode diode laser with distributed Bragg reflectors, an amplifier in tandem with the laser, lateral phase controllers and a detuned second order grating surface output coupler, all on a common substrate. The amplifier is a flared waveguide in one embodiment, and a branching network of single mode waveguides followed by an array of single mode gain waveguides in another embodiment. The diode laser is tunable by means of a separate tuning current applied to the rear Bragg reflector. Tuning the laser wavelength provides, in conjunction with the output coupler, a longitudinal steering of the output beam. The lateral phase controllers are an array of separately addressable electrodes that adjust the optical path length to compensate for phase variation in the amplifiers and also to provide lateral steering of the output beam. An additional embodiment includes a chain of amplifiers and grating output couplers for providing multiple output beams.

Ultrafast, low power, and highly stable all-optical switching in an all polarization maintaining fiber Sagnac interferometer

Abstract: An ultrafast, low-power, and highly stable all-optical switch in a nonlinear Sagnac interferometer is reported. To achieve low-power, highly stable, and walkoff free switching, use is made of a small-core dispersion-shifted polarization-maintaining fiber loop (200 m in length) which has a small group delay difference between the wavelengths of the input signal and the control pulse. To achieve complete polarization stability, a wavelength-sensitive polarization-maintaining fiber coupler is employed. Highly stable and walkoff free all-optical switching is demonstrated at 5 Gb/s. >

Modulation bandwidth of high‐power single‐mode semiconductor lasers: Effect of intraband gain saturation

Abstract: The effect of intraband gain saturation on the modulation bandwidth of single‐mode semiconductor lasers is discussed by using a nonperturbative form of the optical gain that is valid at high‐power levels. The small‐signal analysis of the modified rate equations is used to predict the power dependence of the modulation bandwidth. The results are used to discuss the ultimate modulation bandwidth of InGaAsP distributed feedback semiconductor lasers and its dependence on various device parameters.

Evanescent field amplification in a tapered single-mode optical fibre

Abstract: We describe the fabrication and characterisation of an optical amplifier based on a tapered single-mode optical fibre immersed in a circulating dye solution. Optical gains of over 20 dB have been measured for pump powers of less than one watt, while the onset of gain saturation has been observed for pump powers of only tens of milliwatts.

Solutions of the SBS equations in single mode optical fibres and implications for fibre transmission systems

Abstract: The solutions of stimulated Brillouin scattering (SBS) equations in single mode fibres are reported, and it is shown how the results can be used to enable easy and flexible evaluation of the power budget requirements in optical fibre transmission systems.

Review of quasi-optical gyrotron development

Abstract: There is currently a need for megawatt average power sources of 100–600 GHz radiation for electron cyclotron heating of fusion plasmas. One of the leading candidates for such a source, the conventional wave guide cavity gyrotron,(1) has produced impressive output powers and efficiencies at frequencies up to about 300 GHz. However, this gyrotron configuration is limited at high frequencies by high ohmic heating and problems with transverse mode competition due to the highly overmoded configuration, and with beam collection, since the beam must be collected along a section of the output waveguide. The quasi-optical gyrotron (QOG), first proposed in 1980 by Sprangle, Vomvoridis, and Manheimer,(3) features an open resonator formed by a pair of spherical mirrors instead of a waveguide resonator and has the potential for overcoming each of these limitations. The resonator mirrors can be well removed from the beam-wave interaction region, allowing a large volume for the interaction and low ohmic heating densities at the mirrors. The beam direction is transverse to the resonator so that beam collection is separate from the output waveguide. This geometry is particularly well suited to the use of a depressed collector for electron beam energy recovery. The QOG operates in the lowest-order transverse (TEMool) Guassian mode of the resonator, higher-order transverse modes being effectively suppressed by higher diffraction losses. This paper reviews recent progress toward the development of high-power quasi-optical gyrotrons for ECRH of fusion plasmas. It includes an overview of gyrotron theory in terms of normalized variables as they apply to the quasi-optical gyrotron for operation both in the fundamental and the higher harmonics. Scaling equations for the output power and resonator mirror heating by the RF are given. The design tradeoffs between annular and sheet electron beams are discussed as is the issue of beam space-charge depression in the open resonator. Recent advances in the analysis and design of QOG configurations capable of efficient and stable single-mode operation are discussed, showing the possibility of achieving 50% transverse efficiency in highly overmoded resonators. The application of a depressed collector is discussed as a means of recovering the energy in the axial motion of the spent electron beam and, thus, raising the output efficiency to near the transverse electronic efficiency. The problem of high field magnet design is addressed, for both fundamental and higher harmonic operations, the latter being necessary at very high frequencies. The design equations and tradeoffs are applied to the design of 1-MW, CW quasi-optical gyrotrons operating at 120 GHz, in the first and second harmonic at 280 GHz and in the second harmonic at 560 GHz. The output coupling for these 1 MW designs is 5–7% showing the potential for even higher powers per tube if sheet-beam electron guns can be developed. The estimated electronic efficiency of the fundamental harmonic designs is 23%, which leads to an output efficiency of 47% with the use of a depressed collector with a modest collection efficiency. The peak ohmic heating density is 500 kW/cm2 in all the designs. This leads to resonator mirror separations ranging from 127 cm for 120-GHz design, to 232 cm for the 560-GHz, second harmonic design. Finally, a simple output system composed of'elliptical and parabolic mirrors is described that converts the output radiation from the resonator into a parallel, quasi-Gaussian beam. Experimental programs are reviewed as well, including the recent experiment at the Naval Research Laboratory that produced frequencies ranging from 95–130 GHz and powers up to 150 kW. Operation in a single mode was observed at powers up to 125 kW despite the resonator being highly overmoded. Comparison is made with the theoretically-predicted region of single-mode operation. Recent progress in the experimental characterization of QOG resontors is summarized.

Fiber-optic interferometric sensor for the detection of acoustic emission within composite materials.

Abstract: An optical-fiber Michelson interferometric acoustic emission sensor is described. The sensor uses ordinary singlemode fiber and is embedded in the composite material under test. Signals are demodulated through the active homodyne. This system provides a novel approach for material nondestructive evaluation.

Spectral properties of Rayleigh backscattered light from single-mode fibers caused by a modulated probe signal

Abstract: The statistical properties of Rayleigh backscattering from single-mode fibers are investigated, taking into account the modulation and the coherence of the source light. The autocorrelation function and the power spectral density of the backscattered field and intensity are derived. To describe the noise due to Rayleigh backscattering in bidirectional systems, the spectrum of the interference of backscattered light with reflected source light is also calculated. The measurements confirm the theoretical results. It is shown that, if no optical isolator is used and the laser diode is exposed to Rayleigh backscattering, the interaction between laser and fiber depends strongly on the modulation of the light. >

Single-mode, single-polarization optical fiber

Abstract: A single-mode, single-polarization optical fiber ("PZ fiber") can have a large single-polarization wavelength bandwidth when η (as herein defined) for one symmetry axis is positive when calculated from the refractive index profile determined with one of two orthogonal orientations of polarized light and is negative when calculated from the refractive index profile determined with the other orientation, and for each other symmetry axis of the novel optical fiber η is positive for both orientations of plane polarized light. Preferably the absolute values for η when positive and η when negative are about equal for said one symmetry axis. A preferred PZ fiber can be formed by depositing siliceous layers onto the interior surface of a hollow substrate tube of quartz to provide a preform. After forming two parallel flat faces in its outer surface, the preform is pulled to form a PZ fiber having an elliptical stress-applying region (15).

Theory and fabrication of 4 × 4 single-mode fused optical fiber couplers

Abstract: The theory, fabrication, and performance of monolithic 4 × 4 single-mode fused couplers is reported The field coupling matrix is derived and used to show that a device having equal coupling between the four fibers may be obtained The fabrication of such a device is described, and measurements show that the device exhibits excellent coupling uniformity and low excess loss The wavelength response of a device designed for equal coupling at 153 μm is shown between 12 and 16 μm

Optical transmission of RF subcarriers in adjacent signal bands

Abstract: Apparatus is provided for optically transmitting a plurality of AM modulated RF subcarriers in adjacent octaves. A first laser is modulated to produce a first optical output signal containing a plurality of subcarriers in a first octave. A second laser is modulated to produce a second optical output signal containing a plurality of subcarriers in a second octave. The first output signal is optically filtered to attenuate distortion products falling in the second octave. The second output signal is optically filtered to attenuate distortion products falling in the first octave. The filtered first and second output signals are combined for communication over an optical transmission medium. Where the transmission medium is a single mode optical fiber, filtering can be accomplished using single mode transversal filters.

High performance single mode OTDR using coherent detection and fibre amplifiers

Abstract: A high performance single mode OTDR has been constructed at 1.55 mu m using a DFB laser diode, laser diode pumped fibre amplifiers and a coherent detector, all suited to integration into a robust and portable instrument. A dynamic range of 33 dB has been attained with 1 mu s launch pulses, 1 MHz receiver bandwidth and 10/sup 4/ integrations. This result represents an increase of about 13 dB in performance over the standard version of the OTDR using a laser diode as a light source. It is comparable to the best previously reported value attained using a high power glass laser and a cooled detector.< >