Showing papers on "Mode volume published in 1983"

Fiber-optic gyroscopes with broad-band sources

Abstract: The use of broad-band sources, polarized and unpolarized, in fiber gyroscopes with linearly birefringent fibers, is studied theoretically. Polarization-mode coupling in the fiber is modeled using one-mode coupling center. Gyroscope output equations are obtained which are useful in interpreting an origin of fiber noise in the limiting cases of low and high fiber birefringence. Interference effects in the output are shown to be related to mode coupling at particular locations in the fiber. The extent of these locations is governed by a depolarization length for which numerical estimates are given.

Rayleigh backscattering theory for single-mode optical fibers

Abstract: The theory of backscattering in single-mode optical fibers is described through use of a correlation function for the refractive-index fluctuation in the fiber. A simple formula for the backscattered power is derived using two correlation functions for the Booker–Gordon and Gaussian models. The zeroth-order approximation of the formula, in which the correlation length is much smaller than the spot size of the waveguide mode, coincides with Brinkmeyer’s model. The backscattered power at the input end of single-mode fiber is compared with that for multimode fiber. It is also shown that the backscattered power level at the input end is lower by approximately 55 dB than the input power level.

Single-mode, single-polarization fibers made of birefringent material

Abstract: Optical fibers composed of highly birefringent material are studied. One of the two fundamental modes can be made leaky when the birefringence is sufficiently large. This suggests a novel single-mode, single-polarization fiber design that can be realized by stress-induced birefringence. The leakage rate is calculated by a perturbation method, which accounts for degeneracy between a bound mode and a packet of radiation modes.

Depolarization in a single-mode optical fiber

Abstract: The reduction of the degree of polarization of broad-band light due to propagation in ordinary single-mode fiber is examined theoretically and experimentally. Previous work is extended to account for polarization-mode coupling along the fiber by developing a model for one discrete mode-coupling center and extending it qualitatively to include multiple centers. The existence of nonzero degree of polarization in long lengths of fiber is shown to be due to mode coupling at particular positions along the fiber and the degree of polarization is shown to be related to the degree of coherence associated with the mode-coupling site. The experimental results generally support the model developed.

Sensitive all-single-mode-fiber resonant ring interferometer

Abstract: A new type of fiber interferometer is presented which utilizes a high-finesse all-single-mode-fiber ring resonator. The measured sensitivity to periodic phase shifts induced in the fiber ring is 1.0 \times 10^{-6} rad/ \sqrt{Hz} or better over the 100-Hz-10-kHz frequency range with maximum sensitivity of 1.9 \times 10^{-7} rad/ \sqrt{Hz} at 10 kHz. This sensitivity, while high, is not shot-noise limited and appears to be determined by the spectral bandwidth of the single-frequency laser source.

Measurement of polarization mode coupling along a polarization-maintaining optical fiber using a backscattering technique

Abstract: A new technique for measuring the polarization mode coupling of a polarization-maintaining optical fiber has been proposed that uses a modified optical time-domain reflectometry in which two kinds of backscattered signals coming through each principal axis of the fiber are utilized. This technique shows how the mode coupling occurs along the fiber. The extinction ratio evaluated by the present technique is in good agreement with that obtained by a conventional technique (within ±0.5 dB).

Single mode fiber optic single sideband modulator and method of frequency

Abstract: A fiber optic frequency shifter comprising two waveguides having different indices of refraction. In some embodiments the waveguides are two modes of propagation in one fiber. Plural distributed coupling ridges, or electrodes mounted adjacent piezoelectric materials, are independently driven to apply sinusoidally varying forces to the fiber. In some embodiments, the phase relationship of the driving signals for the electrodes or ridges is such that a travelling acoustic wave is launched in the fiber. In other embodiments, regions of stress in the fiber are created by an acoustic wave coupled into the fiber from a transducer coupled to an acoustic medium surrounding the fiber. The input carrier light is shifted in frequency by the frequency of the acoustic wave.

Bandwidth estimation for multimode optical fibers using the frequency correlation function of speckle patterns.

Abstract: In this paper we present a new method for estimating the bandwidth of multimode optical fibers based on the frequency correlation function of the speckle patterns generated by the interference of fiber modes. This technique, which does not require a pulse or signal generator, can be utilized to estimate the bandwidth of a multimode fiber using a relatively short length of fiber. By applying this method to a test fiber we obtained a bandwidth of ~36 MHz · km which is in relatively good agreement with the ~44-MHz · km bandwidth measured by a conventional pulsed technique.

Low-loss coupling of multiple fiber arrays to single-mode waveguides

Abstract: A method for obtaining permanent low-loss couping between arrys of single-mode fiber and Ti:LiNbO 3 waveguides is described. The technique, based on the use of silicon chip V-grooves, simplifies the coupling problem by simultaneously aligning the entire array and by providing a large surface area for a higher integrity adhesive bond. At \lambda = 1.3 \mu m, we measure an average 1.9-dB coupling loss (exclusive of propagation loss) for the assembled array. The average excess loss due to the fiber array is 0.8 dB. We present an analysis of the effect of various types of array misalignment on coupling efficiency. Angular alignment and array periodicity are found to be critical. If the fiber and waveguide periodicities are matched exactly, the fibers need only be placed within \pm 1.3\mu m of their optimum position to maintain coupling efficiencies greater than 90 percent.

A single-mode-fiber coupler with a variable coupling ratio

Abstract: The tuning of the coupling ratio over a wide range in fused biconical-taper single-mode-fiber couplers by a controlled distortion of the coupling region is demonstrated. The tuning has negligible effect upon excess insertion loss.

Optical power distribution in multimode fibers with angular-dependent mode coupling

Abstract: An expression for the power flow, in multimode optical fibers, is derived, which is mathematically simple and allows a clear physical interpretation of the effects of mode coupling. Coupling strength is assumed to decrease with increasing axial angle. The solution obtained under this assumption is supported by experimental observations. Following the analysis, an experimental procedure to determine the coupling constant is devised. The theoretical results enable the prediction of the loss due to the mode coupling and the transmission bandwidth of the angular multiplexed channels in step-index optical fibers.

Finite cladding effects in W fibers: a new interpretation of leaky modes

Abstract: From recent experimental results, the usual fiber model with infinite outer cladding fails to give an accurate description of the mode behavior beyond cutoff. The finite dimensions of the actual fiber must be taken into account; the leaky mode formalism is then replaced with a theory of coupling between a core mode and discrete, lossy cladding modes. The excitation and attenuation of the modes resulting from this coupling are considered in some detail; this more realistic model allows a better agreement with experiment.

Acousto-Optic Phase Modulator and Frequency Shifter for Single Mode Fibers

Abstract: for several applications as they process the guided liqht directly within the fiber. Two such devices are reported here for nhase modulation and for frequency shiftinq in single mode fibers. The phase modulator is fabricated by nositioning a single mode fiber along the axis of a cylindrical PZT acoustic transducer. The fiber is bonded inside a glass caDillary tube which is, in turn, eDoxied inside the transducinq cy1 inder to give a solid, mechanically stable s tructure. The maximum sensitivity of the phase shifter is 0.058 radians/V-cm at 6.02 YHz. modulation is observed. In the second device, a fiber optic frequency shifter is constructed using a birefringent single mode fiber positioned within two acoustic resonators driven 90' out of phase. The incident wave in one polarization state is counled to a frequency-shifte d wave in the orthoqonal state. The device works out to 5 MHz and 20 dB supnression of one sideband in relation to the o ther one has been obtained. In-line fiber optic devices are attractive

Effect of mode coupling on the total pulse response of perturbed optical fibers.

Abstract: A computer program has been developed to study the total pulse response of optical fibers with profile ripple and central index depressions in the presence of arbitrary mode coupling. We have found that the magnitude of the compression of the total pulse response generated by mode coupling depends significantly on the details of the refractive-index profile of the test fiber.

Bimodal intrusion detection in an optical fiber communication system using graded index fiber

Abstract: An intrusion-alarmed optical communication system in which two separate modal groups are launched into a single graded index transmission fiber. The system includes two light sources and a lens to focus the light sources into the transmission fiber. A spatial filter is used to selectively limit the angles of incidence of the light on the fiber, and operates in conjunction with the lens to launch light from one source at appropriate angles and points of excitation to excite higher-order modes in the fiber, and to launch light from the other source at appropriate angles and points of excitation to excite lower-order modes in the fiber.

Zero dispersion in attenuation optimized doubly clad fibers

Abstract: In a weakly guiding single-mode fiber, there exists an optimum value for the HE 11 mode spot size, which minimizes splicing and microbending losses. Zero dispersion and this optimum spot size for the mode can simultaneously be achieved with any inner-cladding depth; increasing the inner-cladding depth, however, allows smaller dopant concentrations in the core and thus lower intrinsic losses, but this induces an enhanced sensitivity to curvatures and to parameter fluctuations. When the inner-cladding depth is sufficiently high, on the other hand, the fundamental mode has a nonzero cutoff and dispersion can be minimized over a wide spectral range. At \lambda = 1.55\mu m, we determine the optimum value for the inner-cladding depth from criteria of both sensitivity to bending and fabrication tolerances or by requiring a given mode dispersion behavior over a wide spectral range.

External cavity controlled operation of a semiconductor diode gain element in series with an optical fiber

Abstract: The series combination of a semiconductor diode gain element (a diode laser whose end facets have been antireflection coated) and an optical fiber has been placed inside an external cavity and the combined system has lased in a single spectral line whose width was less than the 1.7 \times 10^{-5} -nm (7.5-MHz) resolution of the scanning Fabry-Perot interferometer used in the measurement. This result has been achieved with either a single-mode or a multimode optical fiber in series with the diode gain element and for experiments in which a polarizer, oriented so its polarization is parallel to the diode gain element polarization, was placed at the other end of either fiber. The output of the external cavity is temporally stable as measured both by a 225-MHz-bandwidth detector system and a spectrum analyzer. Over a one minute interval the maximum fluctuation in the output frequency of the external cavity was found to be 2 MHz. Analysis of the threshold behavior of the external cavity for the cases where the multimode or single-mode fiber are in the cavity indicates that the coupling coefficients for either fiber are nearly the same. Without an external cavity, the coupling coefficient from the diode gain element (or from a similar, not anti-reflection-coated, laser) to the multimode fiber is over five times larger than that to the single-mode fiber. These coupling results are explained by postulating that for the multimode fiber, only one or a selected number of the many modes of the fiber can participate in the laser action of the external cavity. It is believed that only these mode(s) are reflected back into the fiber by the spherical mirror in appropriate phase and angle to participate in the laser emission.

Mode coupling loss in single-mode fibers with depressed inner cladding

Abstract: Single-mode fibers with depressed inner cladding show sharp resonant loss peaks near the fundamental mode cutoff wavelength where the core-guided mode starts to become a leaky mode. The magnitude of the loss peak exceeds 200 dB/km and was found to be extremely sensitive to fiber curvature. The phenomenon is attritubed to mode coupling between the core-guided mode and the discrete modes guided in the outer cladding.

High Frequency Response of a Single Mode Fiber Optical Phase Modulator Utilizing a Piezoelectric Plastic Jacket

Abstract: A single mode optical phase modulator utilizing a piezoelectric plastic jacket is studied theoretically. Due to the short wavelength at high frequency and the axial symmetry of the optical fiber, a radially dependent deformation is assumed. Computational results of the optical phase modulation reveal that the optimal performance may be achieved if the modulator is driven at its resonant frequencies. These resonances are caused by the thickness modes of the piezoelectric plastic jacket at lower frequencies and by the breathing modes of the glass fiber at higher frequencies. Experimental measurements (using a Mach-Zehnder interferometric configuration) up to 5 MHz on a 3 cm long sample show reasonable agreement with the theoretical predictions.© (1983) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Birefringence Measurements In Single Mode Optical Fiber

Abstract: Because their cores are not perfectly circular or because of stress, inherent to the structure or externally applied, practical single mode fibers are birefringent. These sources of birefringence are reviewed briefly. A simple model for the fiber consists of a combination of one linearly birefringent element and one circularly birefringent element. Depending on the magnitude of the birefringence, different techniques of evaluating the parameters of the model may be suitable. Several methods appropriate for low and high birefringence fiber are described and some of their advantages and disadvantages outlined. In an idealized single mode optical fiber, in which none of the structural or optical parameters varies with the azimuthal coordinate, all polarization states within the fundamental mode are degenerate; that is, neither the phase nor group velocity depends on polarization. In practice, however, this degeneracy is usually lifted by non-circularity of the core, by inherent stress in the guiding structure, or by stress resulting from bending, pressure, or twisting. Light entering the fiber with an arbitrary polarization state will then be resolved into a pair of orthogonal polarization states that propagate with different velocities. As a phase shift develops between them, the state of polarization will vary periodically along the fiber. The orthogonal states may be linear, circular, or in general elliptical, depending on the specific mechanisms involved.

Hybrid Modes in Circular Cylindrical Optical Fibers

Abstract: The classification of hybrid modes in circular cylindrical optical fibers is studied. It is shown that problems of the mode classifications, i.e., the crossover of the dispersion characteristics and the remarkable changes of the polarization states and the field configurations, are caused by the coupling of the HE-type and EH-type modes.

Optical fiber fabrication process comprising determining mode field radius and cut-off wavelength of single mode optical fibers

Abstract: A single mode optical fiber manufacturing process can be monitored and/or controlled determining the mode field radius and/or the cut-off wavelength of fiber produced by the process, comparing these measured fiber parameter values to predetermined target values, and setting a process step in accordance with the result of the comparison. Both fiber parameters are determined by a procedure comprising measuring, as a function of angle, the far-field power emitted by a test fiber segment, and determining the parameters of a Gaussian function G(θ), typically by maximizing the value of an overlap integral.

Determination of Single-Mode Fiber Refractive Index Profiles by a Propagation Mode Near-Field Scanning Technique

Abstract: The characteristics of an optical fiber, such as spot size, single-mode condition, and dispersion characteristics, are principally determined by its refractive index profile It is important to establish an efficient and accurate method for measuring the refractive index profile For a single-mode fiber, however, the measurement of the profile is very difficult because of its small core diameter and low refractive index difference Several methods were proposed for the measurement of the refractive index profile of single-mode fibers, such as the far-field radiation pattern technique [1],[2], the scattering pattern method [3], the reflection method [4], the focusing method [5], the refracted near-field technique [6], and the interferometric slab method [7]

Optimized Couplers Between Junction Lasers and Single-Mode Fibers.

Abstract: : Single mode fiber output power of over 75 mW cw has been obtained by the use of unique adjustable coupling modules that were developed for this program and that are being delivered to NRL These are the highest powers ever reported coupled between diode lasers and single mode fibers Coupling efficiencies over 25% were obtained in simple butt coupling by taking advantage of the large mode size available from RCA CDH LOC lasers We also observed peak fiber power of over 9 mW with 100 ns 50% duty cycle pulses The miniature models provide relative transverse and lateral motion between fiber and laser with 2000A resolution, allowing fine tuning against long term drifts The coupling, while adjustable, is still stable against moderate acceleration of module and motion of fiber pigtail The adjustments may be adapted to servo control Still higher coupling efficiencies might be expected with cylinder lenses bonded to the fiber Initial experiments indicated the need for further work to achieve accurate and stable attachment of lens to fiber

Constant polarization optical fiber

Abstract: PURPOSE:To discriminate easily the principal axis of an optical fiber in accordance with its outward appearance by forming the section into a noncircular shape which has a prescribed relation to the dirction of a plane of polarization to be maintained CONSTITUTION:The optical fiber is provided with a core 2 in the center and is provided with an elliptic clad 3 around the core 2 By forming the clad into an ellipticity, the lateral pressure applied to the core 2 is changed in accordance with direction to give a difference to propagation speed, and the plane of polarization is maintained The outside shape of the spun optical fiber cannot be circular completely and is elliptic A thin coat layer 5 is formed around this optical fiber to obtain a constant polarization optical fiber 6 whose outside shape of the section is elliptic The fact that the outside shape is elliptic is used to wind the optical fiber 6 around a column 7 so that a normal direction D of the curved surface of the column 7 and a principal axis E of the contstant polarization optical fiber 6 coincide with each other throughout the overall length of the optical fiber Since the optical fiber can be so wound that the normal direction of the curved surface and the principal axis coincide with each other, degradation of an extinction ratio is lessened

Comparison of cutoff wavelength measurements for single-mode waveguides.

Abstract: The intrinsic cutoff wavelength of the LP11 mode is investigated using three different types of measurement for an ITT single-mode fiber. Characterization of the far-field pattern of the LP01 mode gives a cutoff value ~660 nm, a near-field transmission experiment gives ~690 nm, and a refracted power measurement gives ~830 nm. We conclude that the refracted power technique is the best experimental method for the determination of the intrinsic cutoff wavelength of a fiber sample. The effect of the loss of coupling efficiency into the LP11 mode as cutoff is approached on the transmission and refracted power experiments is noted.