Showing papers on "Mode volume published in 2000"

Observation of critical coupling in a fiber taper to a silica-microsphere whispering-gallery mode system

Abstract: We present the observation of critical coupling in a high-Q fused-silica microsphere whispering-gallery mode resonator coupled to a fiber taper Extremely efficient and controlled power transfer to high-Q (∼10^7) resonators has been demonstrated Off-resonance scattering loss was measured to be less than 03% On-resonance extinction in transmitted optical power through the fiber coupler was measured as high as 26 dB at the critical coupling point This result opens up a range of new applications in fields as diverse as near-field sensing and quantum optics

Cladding-mode resonances in short- and long-period fiber grating filters

Abstract: The transmission of a mode guided by the core of an optical fiber through an ultraviolet-induced fiber grating when substantial coupling to cladding modes occurs is analyzed both experimentally and theoretically. A straightforward theory is presented that is based on the calculation of the modes of a three-layer step-index fiber geometry and on multimode coupled-mode theory that accurately models the measured transmission in gratings that support both counterpropagating (short-period) and co-propagating (long-period) interactions. These cladding-mode resonance filters promise unique applications for spectral filtering and sensing.

Optical bend sensor based on measurement of resonance mode splitting of long-period fiber grating

Abstract: We report an effective new method of realizing optical bend sensing based on the measurement of bending-curvature encoded resonance mode splitting of long-period fiber grating. The bending induced mode splitting exhibits a near-linear response and the bending sensitivity achieved by this method is nearly four times higher than the previously reported wavelength shift detection method. The evolution of the transmission loss under bending appears dependent on the initial mode coupling strength.

Fiber mode coupling in transmissive and reflective tilted fiber gratings.

Abstract: Whereas core-mode reflection and core-mode-to-radiation-mode coupling in tilted fiber Bragg gratings is well understood, as is coupling between a core mode and higher-order core and cladding modes in untilted gratings, here we analyze in detail the coupling among core modes and cladding modes in reflective and transmissive tilted fiber gratings. We show that strong coupling between an LP(01) core mode and the exact (1m) cladding modes occurs in a transmissive tilted grating for nearly any tilt angle except angles close to 90 degrees, whereas the LP(01)-to-(lm) cladding mode coupling (l not equal 1) is appreciable only for tilt angles just below 90 degrees (approximately 88 degrees). In a reflective grating, strong coupling between the LP(01) core mode and the exact (1m) cladding modes occurs only for angles less than approximately 5 degrees, whereas coupling to (1m) cladding modes for m > 1 occurs only for angles greater than approximately 5 degrees. Coupling among bound core modes exhibits a similar behavior, except that in general the coupling is stronger. Experimentally we show coupling to both higher-order bound core modes and cladding modes in a transmissive tilted grating at visible and near-infrared wavelengths.

Measurements of beat length and perturbation length in long single-mode fibers

Abstract: Experimental results of measurement of the beat length and the differential group delay of several types of long single-mode fiber are presented. The proposed measurement technique is based on a polarization-sensitive analysis of the backscattered signal and allows one to calculate the correlation length of the random birefringence affecting the fiber.

High-resolution measurement of the fiber diameter variations using whispering gallery modes and no optical alignment

Abstract: Light is coupled from the guided mode along one tapered optical fiber into the whispering gallery modes around the circumference of another. Small diameter variations in the second fiber are measured by tracking, at different points along it, the wavelengths at which these modes exist. The measurement does not require any optical alignment, and its resolution can be 1 part in 10 000 or better.

Coupling system to a microsphere cavity

Abstract: A system of coupling optical energy in a waveguide mode, into a resonator that operates in a whispering gallery mode. A first part of the operation uses a fiber in its waveguide mode to couple information into a resonator e.g. a microsphere. The fiber is cleaved at an angle Φ which causes total internal reflection within the fiber. The energy in the fiber then forms an evanescent field and a microsphere is placed in the area of the evanescent field. If the microsphere resonance is resonant with energy in the fiber, then the information in the fiber is effectively transferred to the microsphere.

Vector description of higher-order modes in photonic crystal fibers

Abstract: We extensively study the propagation features of higher-order modes in a photonic crystal fiber (PCF). Our analysis is based on a full-vector modal technique specially adapted to accurately describe light propagation in PCF's. Unlike conventional fibers, PCF's exhibit a somewhat unusual mechanism for the generation of higher-order modes. Accordingly, PCF's are characterized by the constancy of the number of modes below a wavelength threshold. An explicit verification of this property is given through a complete analysis of the dispersion relations of higher-order modes in terms of the structural parameters of this kind of fiber. The transverse irradiance distributions for some of these higher-order modes are also presented, showing an excellent agreement with recent experimental measurements. In the same way, the full-vector nature of our approach allows us to analyze the rich polarization structure of the PCF mode spectrum.

Single mode optical fiber coupling systems

Abstract: Coupling devices (90, 140) for coupling a lens (8, 30, 72, 96) to a single mode optical fiber (4, 104, 144, 184) are disclosed. The lens assembly is the coupling device and includes a single mode optical fiber (4, 104, 144, 184), a lens (8, 30, 72, 96), and a locating tube (6, 114, 182).

Gigahertz-repetition-rate mode-locked fiber laser for continuum generation.

Abstract: We report direct generation of <500‐fs pulses at a 1-GHz rate from a self-starting passively mode-locked fiber laser by regeneratively synchronizing the pulses with a phase modulator. The pulses are amplified and passed through a dispersion-decreasing fiber and a normal-dispersion supercontinuum fiber. The resulting continuum is wider than 350 nm.

Propagation of the fundamental mode in curved graded index multimode fiber and its application in sensor systems

Abstract: A novel principle of light transmission through very small radius bend in optical fibers is presented. The potential applications of the proposed structure are fiber optic sensors and other fiber optic systems. The design makes use of graded index multimode fiber as transmission medium. However, the feed to the multimode fiber is through a single mode fiber to ensure that only the lowest order spatial mode is launched. Similarly the receiver is also coupled to the sensing element through a single mode fiber. The fundamental mode within graded index multimode fiber proves to be very insensitive to macrobends if bend radius is larger than certain critical value. If bend radius is reduced below critical value the loss increases very rapidly and this allows for construction of relatively sensitive macrobend fiber optic sensor. This work presents a quantitative theoretical model of the proposed structure and a detailed experimental investigation of structure with possibilities for its practical applications.

Investigation of mode coupling in step index plastic optical fibers using the power flow equation

Abstract: Using the power flow equation, we have examined the mode coupling caused by intrinsic perturbation effects of the step index plastic optical fiber. A numerical solution has been obtained by the explicit finite difference method. Results show the state of mode coupling along the fiber. They indicate that the equilibrium mode distribution is achieved at much shorter lengths compared to the case with glass fibers.

Excitation of cladding modes in photonic crystal fibers by flexural acoustic waves.

Abstract: We report the excitation by flexural acoustic waves of an individual cladding mode in a single-mode photonic crystal fiber. The propagation constant and the field distributions of the mode have been investigated by use of this technique. The results give the basis for developing a family of acousto-optic devices based on photonic crystal fibers.

Polarization mode dispersion emulator

Abstract: We investigate a new technique to realistically emulate polarization mode dispersion. We demonstrate that 15 sections of polarization-maintaining fiber with randomly rotatable connections emulates an almost ideal Maxwellian differential group delay (DGD) distribution, whereas fixed connections is inadequate.

Experimental and scalar beam propagation analysis of an air-silica microstructure fiber.

Abstract: We study the higher order guided modes in an air-silica microstructure fiber comprising a ring of six large air-holes surrounding a Germanium doped core. We characterize the modes experimentally using an intra-core Bragg grating. The experimentally observed modes are then accurately modeled by beam propagation simulations using an index profile similar to the observed fiber cross section. Theory and experiment confirm the presence of “inner cladding” modes with approximate cylindrical symmetry near the core, similar to conventional cladding modes, but which strongly exhibit the symmetry of the microstructure at large radius. Such modes are useful in fabricating robust tunable grating filters and we show that the Bragg grating is a useful diagnostic to measure their effective indices and intensity profiles.

High order spatial mode optical fiber

Abstract: The invention relates to method and apparatus for transmitting an optical signal having optical energy substantially in a high order spatial mode. The optical waveguide, in one embodiment, includes a few mode fiber designed to have specific transmission characteristics for supporting the single high order spatial mode, and the few mode fiber transmits the single high order spatial mode. The optical waveguide, in one embodiment, has a dispersion and a dispersion slope for a given transmission bandwidth. Another aspect of the invention includes a method for transmitting an optical signal having optical energy substantially in a single high order spatial mode. The method includes the steps of providing a few mode fiber, which supports optical energy in the single high order spatial mode. In one embodiment, the single high order spatial mode is the LP 02 spatial mode In another embodiment, the few mode fiber supports an optical signal having optical energy having less than twenty spatial modes.

Near-field scanning optical microscope with a high Q-factor piezoelectric sensing element

Abstract: A perpendicular-mode near-field scanning optical microscope (NSOM) utilizing a piezoelectric micro tuning fork as its height-sensing element is described. The present invention provides a method and apparatus for modifying and attaching an optical fiber to the tuning fork that allows the assembly to retain Q-factors up to 9000, substantially higher than those described so far in the literature for tuning-fork-based instruments. The method involves reducing the diameter of the cladding of the optical fiber down to the 17-25 µm using several chemical etching steps, before the fiber is attached to the tuning fork. A sharp upturn in the Q-factor is observed when the fiber diameter d drops below ~25 µm. An analysis, which shows that the stretching force constant of a bent fiber is proportional to d 4, is used to account for the great sensitivity of the Q-factor to the fiber diameter. The high Q-factors resulted in improved force sensitivity and allowed us to construct a perpendicular mode instrument without the use of additional dithering piezoelements. An improved NSOM operating in the shear force mode is also provided by thinning the optical fiber length running down the length of one of the tines to a thickness in the range from about 50 to 60 µm.

Reducing cross gain modulation in the semiconductor optical amplifier by using wavelength modulated signal

Abstract: A novel dual-input Mach-Zehnder modulator was used in conjunction with a wavelength modulation scheme to remove the cross-gain modulation in the semiconductor optical amplifier (SOA). Eight 10 Gb/s wavelength-division-multiplexed channels are transmitted over 75 km of dispersion-shifted fiber with a semiconductor optical amplifier as an in-line amplifier, with less than 1.5-dB penalty.

Methods and apparatus for mechanically enhancing the sensitivity of longitudinally loaded fiber optic sensors

Abstract: An optical fiber is provided with a Bragg grating formed along a portion of its core and a mechanical structure arranged adjacent to the Bragg grating for amplifying longitudinal strain on the fiber in the vicinity of the grating. The mechanical structure is designed to convert ambient pressure into longitudinal strain on the fiber in the vicinity of the grating and to allow the fiber to pass through the structure so that several pressure measuring apparatus may be arranged along a single optical fiber. An intermediate structure is provided between the fiber and the mechanical structure for minimizing buckling of the fiber. The methods of the invention include converting pressure into longitudinal strain on an optical fiber, amplifying the effect of pressure on the longitudinal strain, measuring pressure by determining the spectral location related to peaks (or minimums) of light reflected from an optical grating subjected to longitudinal strain.

Refractive index profile of polyethylene fiber using interactive multiple‐beam fizeau fringe analysis

Abstract: A multiple-beam interference Fizeau fringes technique is used to measure the refractive index profile of drawn polyethylene fiber. The interference fringe shift in the fiber region has been analyzed automatically using an interactive algorithm. The method takes into consideration the refraction of the light beam when crossing the fiber. Plane polarized light vibrating parallel and perpendicular to the fiber axis are used to obtain the refractive index profiles of both cases. These profiles are used to determine some optical parameters such as the birefringence, the optical orientation function, the polarizability per unit volume, and the value Δα/3αo, which related to the material structure. The reliability of the method is tested considering the results of drawn polyethylene fiber samples using the manual technique. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 3099–3106, 2000

Dispersion compensating module and mode converter, coupler and dispersion compensating optical waveguide therein

Abstract: A dispersion compensating module, mode converter, coupler and dispersion compensated optical fiber therein. The dispersion compensating fiber has a plurality of core segments, the refractive index profile being selected to exhibit properties such that an LP02 mode at 1550 nm may be propagated a distance (generally 0.5-3.0 km), upon conversion to LP02, to compensate for dispersion of a length of transmission waveguide preferably greater than 25 km propagating in an LP01 mode. In another embodiment, the dispersion compensating module has a mode converter having a reflective fiber grating for converting a first to a second mode interconnected to a dispersion compensated fiber propagating in the second mode. The mode converter has a coupler adapted to operatively couple light propagating in a first mode from a first fiber into a second, and a reflective fiber grating operatively coupled to the second fiber; the grating being capable of converting light from the first into the second mode. According to another embodiment, an optical fiber coupler is provided having a first fiber with a first propagation constant in a first mode, and a second fiber within the coupler having a second propagation constant, the second fiber including a necked-down portion which is formed prior to fusion of the fibers, the necked-down portion being formed such that the local propagation constant of the second fiber substantially matches the first propagation constant thereby enhancing first mode coupling.

Method of qualifying a multimode optical fiber for bandwidth performance

Abstract: A method is disclosed for qualifying a multimode optical fiber 150 for bandwidth performance when used with a particular laser source. The method combines the modal power distribution (MPD) excited by a particular laser source with the differential mode delay (DMD) characteristic of the fiber. The DMD of the fiber is measured by injecting test pulses into one end of the fiber and detecting the resulting output pulse(s) at the other end. The test pulses are adapted to excite only a small number of the modes supported by the fiber. The test pulses are scanned across the core of the fiber at close intervals with the output pulse(s) stored at each radial position. A weighted sum of the output pulses is formed to determine a time-domain impulse response, where the weighting used corresponds to the MPD excited by the laser source. Bandwidth is then determined by standard methods for transforming the impulse response into the frequency domain. In one embodiment of the invention, a weighted sum of the DMD data is used in the determination of bandwidth; whereas in another embodiment of the invention, a deconvolution algorithm is applied to the DMD data to obtain modal delay times for each of the mode groups of the fiber, which are then combined with the MPD excited by the laser source.

High performance fused-type mode selective coupler for two-mode fiber devices

Abstract: We demonstrate a high performance fused-type mode selective coupler that couples the LP11 mode of a two-mode fiber and the LP01 mode of a matching single-mode fiber, the phase-matching condition was satisfied by etching and pre-pulling of a two-mode fiber, the excess loss and the maximum coupling ratio of 0.15 dB and 92%, respectively, are achieved.

Noise reduction technique for cladding pumped optical amplifiers

Abstract: Systems and methods for suppressing cladding modes at the signal wavelength in an optical fiber system utilizing an optical amplifier without perturbing the pump wavelength. An outer cladding of the gain fiber is removed providing a short section of a single mode fiber that does not guide the higher order modes, thus attenuating the higher order modes. Therefore, this system eliminates unwanted noise. In an embodiment, the modes are filtered at the end of the gain section of the optical fiber before splicing to an input fiber. Hydro florate etching can be utilized to remove the outer cladding. The resulting fiber maintains fundamental mode propagation and no mode mixing occurs while losing the higher order modes eliminating the noise.

Thermally insensitive pressure measurements up to 300 degree C using fiber Bragg gratings written onto side hole single mode fiber

Abstract: A thermally insensitive pressure measurement up to 300 degree C was demonstrated utilizing fiber Bragg gratings written onto a side-hole single mode fiber. The resulting temperature sensitivity is about 300 times lower than normal FBGs.

Multimode optical fiber having function to remove higher order mode

Abstract: PROBLEM TO BE SOLVED: To obtain an optical fiber having a large effective core area, low loss, small absolute value of dispersion and small bending loss susceptibility by controlling the difference in the propagation constant between the lowest mode and the second-order mode to be twice or more of the difference in the propagation constant between adjacent modes of higher-order modes. SOLUTION: The distribution structure of the refractive index is designed in such a manner that when light signal enters, three or more modes of linearly polarized waves are present as the propagation modes. In this state, the difference in the propagation constant between LP01 mode and LP11 mode is specified to be twice or more, preferably three times or more, of the difference in the propagation constant of the LP11 mode or higher-order mode and the adjacent higher mode. As a result, this structure enables such an arrangement of modes that only the LP11 mode or higher-order modes are coupled to the crack mode and that only the LP01 mode substantially propagates after the light propagates at a proper distance. If the difference is less than twice, proper attenuation for only the higher-order modes is impossible and a single mode transmission is made difficult.

Large effective area dispersion compensating fiber for cabled compensation of standard single mode fiber

Abstract: Production of wideband dispersion and dispersion slope compensating fiber with 35 /spl mu/m/sup 2/ effective area and low attenuation is demonstrated. This fiber is optimized for cable deployment in a 1:1 length ratio with standard single mode fiber.

Photonic crystal distributed feedback fiber lasers with Bragg gratings

Abstract: Two new types of optical fibers, where air-holes are running down their length, are considered for making fiber lasers with Bragg gratings. The mode areas for pump and signal in these fiber lasers may be either larger or smaller compared to the corresponding mode areas for fiber lasers based on standard step index fibers. This makes possible realization of fiber lasers with a low pump threshold (small mode area), and fiber lasers suitable for high-power applications (large mode area).

Optical transmitter having optically pumped vertical external cavity surface emitting laser

Abstract: An optical fiber transmitter comprises an optically pumped single mode MQW VECSEL for emitting an information-carrying laser beam and has an external cavity length defining a comb of optical modes, each mode corresponding to a channel wavelength of an optical telecommunications system having plural optical channels. A semiconductor structure of the VECSEL has an optical-pump-excited multiple quantum well (MQW) homogeneously broadened gain region active over a band which is less than mode-to-mode spacing, the gain region being tunable to hop from a first mode to an adjacent second mode. A tuning arrangement tunes the VECSEL from mode to mode thereby to select each one of the plural optical channels. An optical modulator adds modulation to a beam emitting from the laser to provide the information-carrying laser beam, and a coupler couples the information-carrying laser beam into an optical fiber of the optical telecommunications system. A calibration method based on detecting inter-mode optical transitions (mode hopping) is described.