Showing papers on "Mode volume published in 1979"

Polarization optics of twisted single-mode fibers

Abstract: In twisted single-mode optical fibers the polarization of light is affected by an elastooptically induced optical activity and by a modification of any linear birefringence present. These effects are discussed theoretically and demonstrated experimentally. The activity/twist ratio is α/τ ≃ 0.13 … 0.16 universally in weakly guiding silica fibers. Twisted fibers may be used as polarization rotators. A fiber with a ±68° double twist operates as a fast/slow mode interchanger, suitable for delay equalization.

Single-mode optical waveguides.

Abstract: An efficient and powerful technique has been developed to treat the problem of wave propagation along arbitrarily shaped single-mode dielectric waveguides with inhomogeneous index variations in the cross-sectional plane. This technique is based on a modified finite-element method. Illustrative examples were given for the following guides: (a) the triangular fiber guide; (b) the elliptical fiber guide; (c) the single material fiber guide; (d) the rectangular fiber guide; (e) the embossed integrated optics guide; (f) the diffused channel guide; (g) the optical stripline guide.

Polarization stabilization on single‐mode fiber

Abstract: The state of polarization at the output end of a long nominally circular single‐mode optical fiber is stabilized by an active control system. It contains a polarimeter and two electromagnetic fiber squeezers which introduce variable amounts of stress birefringence directly into the fiber, compensating for the effects of changing intrinsic fiber birefringence.

Optical fiber acoustic sensor utilizing mode-mode interference.

Abstract: A method for detecting sound using a single step-index multimode fiber is presented. The detected signal results from differences in acoustically induced phase shifts between two different waveguide modes propagating in the fiber. The relative sensitivity of this technique compared with a two-path interferometer was experimentally determined and agreed with that calculated using the fiber parameters. Because the sensitivity of this approach is proportional to the difference in propagation constants for modes in the fiber, it is approximately 10−3 less sensitive than the single-mode interferometer arrangement.

Phase-nulling fiber-optic laser gyro

Abstract: A fiber-optic gyro is described that employs closed-loop phase compensation. Preliminary experimental results are reported of the sensing of rotation rates down to 0.5 degrees /sec for a 135-mm-radius, 100-m-length fiber coil.

Calculation of bandwidth from index profiles of optical fibers. 1: Theory

Abstract: This paper describes a method for calculating the impulse response and bandwidth of multimode optical fibers from measured refractive-index profiles obtained either from the fiber itself or from its preform. The computational method is based on the WKB solution of the guided-mode problem. First, the pulse delay time of each mode is calculated. The different arrival times of impulses carried by the modes are then used to construct the shape of the impulse response curve whose Fourier transform may be used to predict the signal bandwidth of the multimode fiber. By omitting mode groups or weighting the power distribution among the modes, the influence of certain mode groups on pulse distortion can be studied separately. Dispersion of the host material and of one dopant can be taken into account. The method has been used to study the effects of deviations from the desired perfect index profile and the influence of a central dip. The practical value of the computer program is its ability to predict fiber performance from index measurements made on preforms even before the fiber is drawn.

Mode conversion and optical filtering of obliquely incident waves in corrugated waveguide filters

Abstract: By employing a coupled-mode analysis, we reveal the presence of strong coupling between TE and TM modes in a periodically corrugated optical-waveguide when the wave propagates obliquely with respect to grating vector. The evidence of such mode coupling has been obtained by the polarized transmission and reflection experiment; then the measured strengths of coupling among various modes are successfully compared with calculated values. The effect of such coupling on the performance of a frequency demultiplexer is discussed with some preliminary experiments.

Measurement of refractive-index profile and transmission characteristics of a single-mode optical fiber from its exit-radiation pattern

Abstract: A new exit-radiation pattern method for measuring the refractive-index profile of a single-mode fiber is proposed and used experimentally. In this method the profile is computed from the far-field exit-radiation pattern of the HE11 mode at the end of the single-mode fiber. This method can be applied to a cabled fiber, and the propagation constant, field profile, and group delay of the HE11 mode, and the single-mode limit can be obtained as well as the index profile. The principle, computer simulations, experimental setup, and experimental results are first described. The profile obtained shows a good agreement with that obtained from the interference fringe pattern measured before drawing the fiber. Computations of the group delay of the HE11 mode and the single-mode limit from the measured exit-radiation pattern are discussed.

Single-mode fiber-optical polarization rotator.

Abstract: A birefringent single-mode optical fiber is twisted mechanically in alternating sense on successive half-wave fiber sections. This arrangement, applicable in-line to a fiber, permits continuous adjustment of the state of polarization at the fiber output.

Cut-off wavelength measurements for single-mode optical fibers.

Abstract: A new technique has been proposed for direct measurement of the cut-off wavelength, at which the first higher-order mode disappears. It uses a change of a near-field pattern of a fiber, which is excited by a variable wavelength source. The cut-off wavelength can be measured with +/-5-nm accuracy. The most suitable fiber length for precise measurement is 10-20 mm. It is found, furthermore, that the first higher-order mode under the condition near cut-off rapidly attenuates because of waveguide imperfections, in which the loss due to core-cladding boundary distortions is the most dominant.

Determination of modal power distribution in graded-index optical waveguides from near-field patterns and its application to differential mode attenuation measurement

Abstract: A technique is introduced that determines power distribution in fibers from the measured near-field pattern, assuming that: (1) the optical power distributes uniformly among degenerated modes with the same propagation constant, (2) enough modes are excited to ensure the validity of calculation by geometrical optics, and (3) the phase of each propagation mode has no correlation Experiments verifed that the fibers have the function of flattening power distribution among modes with the same propagation constant This fact shows that assumption (1) does not severely limit the applicability of the technique Wave optical calculation is done to determine the numbers of modes that must be excited to satisfy assumption (2) As an example of application of the technique, differential mode attenuation of graded-index fibers is determined from longitudinal variation of the measured near-field pattern

Effects of profile deformations on fiber bandwidth.

Abstract: Any deviation of the refractive-index profile of an optical fiber from its optimum shape dramatically lowers the fiber bandwidth. In this paper we study the influence on bandwidth of a number of refractive-index deformations commonly encountered in preform manufacture. The deformations include sinusoidal ripples near the fiber axis, a central dip, and a departure from optimum shape near the core–cladding boundary. We also consider a deformation in the shape of a sinusoidal half-period whose length covers one fifth of the core radius. Placing this bulge deformation at five different radial positions between the axis and the core–cladding interface we investigate how the position of an index deformation affects the bandwidth on the assumption that all modes are equally excited. It is demonstrated that the theoretically realizable bandwidth of about 11,000 MHz·km is reduced to several hundred MHz·km for many of these deviations. Comparison is made with an experimentally measured profile.

Optical loss measurement in graded-index fiber using a dummy fiber.

Abstract: The utility of an auxiliary fiber, called a dummy fiber, is investigated for optical fiber loss measurements. The dummy fiber is spliced and used to excite the test fiber. Excess loss caused by undesirable modes is found to be reduced to less than 0.05 dB by using a 500-m dummy fiber and choosing the test fiber cut length to be 2 m for reference. Loss linearity to the fiber length is examined on 6-km spliced fibers, and satisfactory agreement is obtained between the total loss and the sum of the individual fiber and splice losses.

Determination of mode power distribution in a parabolic-index optical fibers: Theory and application

Abstract: A novel measurement method for determining mode power distribution in a multimode parabolic-index fiber is developed. Using this method, the mode power distributions are obtained in terms of the principal mode number by numerically processing the measured Fraunhofer diffraction patterns of the near-field patterns on an output fiber endface. As an example, differential mode attenuation of a multimode parabolic-index fiber is measured. It is confirmed experimentally that the method is practically applicable to the mode power distribution measurements in a parabolic-index fiber.

Nondestructive determination of refractive index profile of an optical fiber: backward light scattering method.

Abstract: A new nondestructive technique is presented for determining the refractive index profile of an optical fiber from its backscattered pattern arising from a normally incident laser beam to the fiber axis. The proposed method requires no sample preparation or index matching liquid. The principle of the method is to construct a deflection function from the measured pattern. The index profile can then be determined by the inversion of an Abel integral equation. Good agreement is obtained between the index profile determined by this technique and that measured by the near-field scanning technique.

Feedback control of lasers in fiber optic systems

Abstract: A feedback control arrangement in a fiber optic lightwave transmitter compensates for changes in the average power coupled from a laser into a fiber due to certain displacements of the fiber relative to the laser. The arrangement includes a laser (10) and an optical fiber (28) having a beveled end face (30) oriented to couple a major portion (13) of the laser beam into the fiber. The position of the fiber relative to the beam axis (12) is sensed by a pair of photodetectors (20, 22) positioned on opposite sides of the beam axis so as to receive other portions of the beam (15, 17) not coupled into the fiber. A feedback circuit (24) is selectively responsive to the smaller of the photodetector outputs for controlling the drive current to the laser so as to maintain the average power coupled into the fiber essentially constant.

Experimental verification of modal dispersion free characteristics in a two-mode optical fiber

Abstract: Group delay time difference between LP 01 and LP 11 modes has been measured in a step-index type optical fiber over a two-mode region. The measured results have revealed that there exists a wavelength at which the group delay times of the two modes are coincident.

On the mechanism of transduction in optical fiber hydrophones

Abstract: The receiving sensitivity of optical fiber hydrophones is determined in part by the rate of change in the optical path length of the sensing fiber with respect to changes in the external pressure on the fiber. These changes in the optical path length are a result of the induced changes in index of refraction of the fiber (photoelasticity) and changes in the physical length of the fiber (elasticity). These two properties are analyzed to determine their individual impacts on the hydrophone sensitivity. It is shown that both of these two effects act to increase the optical path length of the fiber as it is subjected to an increasing external pressure. Therefore, the two effects act in concert with one another, not against one another, in their effect on hydrophone sensitivity.

Single-mode propagation in 2-mode region of optical fibre by using mode filter

Abstract: The effect of a mode filter using fibre bend has been clearly shown experimentally. The mode filter effectively removes the LP11-mode, and has a negligibly small effect on the attenuation of the LP01-mode. The single-mode propagation was attained at the V-value of 2.8 by using the filter.

Multimode fiber with z-dependent index profile

Abstract: Increased mode dispersion in graded-index, multimode optical fibers due to defects in the index profile can be significantly reduced if the average defect is caused to vanish over the length of the fiber. For fibers with non-optimum α-values, minimum impulse response can be approached provided α varies along the fiber, such that the average value of α is equal to αopt. The effect of fluctuations superimposed upon the index profile are minimized by introducing an odd number of phase reversals in the fluctuations equally spaced along the fiber.

Method and apparatus for side launch excitation of selected modes in graded-index optical fibers

Abstract: An optical fiber having a parabolic-index profile is etched to a taper below the cladding layer. A prism contacts the taper along its length within the etched portion of the fiber, and coherent light is transmitted through the prism into the fiber. Selective control of the light beam entry angle, entry position along the taper, and control of the etching depth allows excitation of specific modes within the fiber and maximization of coupling efficiency.

Exact expressions for scalar modal eigenvalues and group delays in power-law optical fibers

Abstract: Considerable use is made of power-law refractive-index profiles in fiber optics. These profiles have simple scaling properties that are exploited in this paper to obtain formulas for the exact modal propagation constants and group delays. When an infinite profile is assumed, the results of the exact, WKB, and geometric optics theories are all shown to agree. This conclusion remains valid when linear material dispersion is included. The effect of the cladding is discussed. The results are correct for noncircular fibers of power-law type.

Modal treatment of an optical fiber with a modified hyperbolic secant index distribution

Abstract: The hyperbolic secant index distribution is known to be optimal for a two-dimensional waveguide, in the sense that all guided modes propagate at the same group velocity. We solve the three-dimensional wave equation for a slightly modified hyperbolic secant index profile. By calculating the propagation constants of the guided modes we can estimate the impulse response of the fiber. We show that in the hyperbolic secant fiber the effective pulse broadening is twelve times smaller than in the parabolic profile, and somewhat narrower than that produced by the optimal a-power profile.

Super wide band light transmitting system

Abstract: There is inserted between a light source and an input of an optical fiber a mode filter which changes the phase of light to be propagated continuously or discontinuously so as to convert guided modes of lower azimuthal number into those of higher numbers in a light transmitting system which employs a graded index optical fiber with a valley at the core-cladding boundary. This reduces the mode-delay differences for all modes including those which are close to cut off so that an optical communication system having a very wide bandwidth is obtained.

Optical amplification for the fiber interferometer gyro

Abstract: An improvement for a fiber interferometer gyro provides for greater rotation rate sensitivity. The gyro has a single mode fiber coiled about an identical area a great number of times. A single external laser sends pulses of energy which are split and passed in opposite directions through the coiled fiber and a pair of optical amplifiers inserted to amplify the energy pulses. This permits the length of the coiled fiber to be increased and, hence, provides for greater interferometer sensitivity.

Diameter determination of graded-index optical fibers from backward-scattered pattern.

Abstract: The backscattered pattern of an optical fiber with arbitrary refractive-index profile is used to determine the fiber diameter. Two methods are presented: the fiber diameter can be determined either by noting the position of a particular fringe maximum or by counting the total number of fringes in the pattern. This method is fast in that it does not require any mechanical moving part, and the measurement error is 1%. This method is based on the prior knowledge of the refractive-index profile of the fiber, but it is shown that the error incurred by assuming the wrong profile is less than 1%. It is also shown in this paper that the method can be used to monitor the diameter variation while the fiber is being pulled. This is done by tracking the displacement of a particular fringe. It is shown that this method is also applicable to fibers with elliptical cross section.

Interferometry Through Single-Mode Optical Fibers

Abstract: A single-mode optical liner can De used as a leg of an interterometer, making possible new applications of interferometry. Since this kind of fiber does not allow higher-order modes to propagate, it acts as a spatial filter and provides a smooth wavefront at its output end. A practical method of providing optically good in-put and output faces for a fiber core only a few micrometers in diameter is described, including a means of stripping away cladding modes. The problem of coupling from a laser into a single-mode fiber is discussed, including optimal matching of a gaussian beam to the Bessel function field distribution of the HEll mode. Theoretical coupling efficiency can be as high as 99.7 percent, and experimental efficiency is 70 percent, not corrected for Fresnel reflections. Experimental results are presented showing the change in the optical length of a fiber with temperature and the use of single-mode fibers in two types of interferometer, a Fabry-Perot etalon and an unequal-path Mach Zehnder interferometer with 128 meters of optical fiber in one of the paths.© (1979) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

On local normal modes in optical fiber and film waveguides

Abstract: It is rediscovered that some early works on conventional waveguides, including several published papers by the author, are applicable to optical fiber and film waveguides. This paper shows that the coupling coefficients for ideal modes, for local modes and for superlocal modes are all related by similar matrices in the sense of Lowey, and as far as discrete modes are concerned, the "equivalence" expressions derived by Marcuse are special cases of our general results. Physical arguments will illustrate the reasonableness, under certain restrictions, of extending the applicability of the said relations to eases which also include a continutun of modes.