Showing papers on "Radiation mode published in 1978"

Polarization mode dispersion in single-mode fibers

Abstract: In real single-mode optical fibers, imperfections cause the two possible polarizations to propagate at different phase velocities This birefringence leads to different group velocities We have measured the resulting mode dispersion in short fiber lengths (05-25 m) from the depolarization of broad-bandwidth light In a typical fiber we found 30 psec/km at 069-microm wavelength, in good agreement with the observed birefringence The effect of mode dispersion can be compensated by a +/-68 degrees double twist midway along the fiber, interchanging the fast and slow modes

Coupled mode analysis of dielectric planar branching waveguides

Abstract: The mode conversion behavior of planar dielectric branching waveguides is studied as a function of branching angle, propagation direction of modes, and thickness ratio of branches The multimode coupled equation and new step approximation method of branch taper are introduced, which give the accurate analysis of mode behavior in the branching waveguide of a large taper angle

Selective mode excitation of graded index optical fibers.

Abstract: A method is described permitting excitation of a small number of modes in graded index fibers, the order of the launched mode being easily varied and determined. Using this launching technique, it is possible to determine the index profile and index difference of graded core fibers; results are compared with those obtained by other techniques. Other important fiber parameters, such as the differential attenuation and the differential propagation delay time as a function of the mode parameter, are also obtained, giving an insight into the fundamental propagation characteristics of the fiber.

Field deformation in a curved single-mode fibre

Abstract: Simple formulae are given for the field displacement caused by bending, the change in mode volume and for the mode coupling loss which arises at transitions between straight and curved sections of single-mode optical fibre. These formulae are in good agreement with other, more exact, theories.

Optical waveguide mode scrambler

Abstract: An optical waveguide mode scrambler including a length of high-alpha optical waveguide optically coupled to a length of low-alpha optical waveguide which provides efficient spatial and angular mixing of light propagating therethrough is described

Transmission characteristics of a two-mode optical waveguide

Abstract: Transient effects of signal transmission in a multimode fiber are investigated in a simple two-mode waveguide model. This paper discusses the condition on which the impulse response or base-band frequency response is defined. Several closed form expressions of the impulse, or frequency, response are derived and their physical implications discussed. The effect of mode scrambling at fiber splices are elucidated in view of the temporal (or phase) difference between modes.

An optical waveguide TE‐TM mode splitter

Abstract: This letter reports an optical waveguide TE‐TM mode splitter using coupling between nonidentical waveguides. The experimental mode splitter was fabricated from glass compositions, and TE0‐TM0 mode splitting has been demonstrated.

Single-mode optical-waveguide fiber coupler.

Abstract: A single-mode fiber coupler to the Ti diffused LiNbO3 strip waveguide has been devised. The influences of three axial displacements and two angular misalignments on the coupling efficiency have been investigated at 6328-A wavelength. The coupler has a special feature wherein coupling degradation caused by fiber displacement after connection can be recovered to the initial state. The total optical insertion loss is 3 dB after fixing the fiber to the LiNbO3 strip waveguide, which is 4 μm wide and 8 mm long.

Single mode fiber-to-channel waveguide through-line coupler

Abstract: A single-mode optical fiber to a single-mode optical channel waveguide end-fire coupler. A single fiber held within a capillary tube and positioned by a micropositioner is adjusted for greatest light output and then secured by an epoxy to one end of a channel waveguide. The two elements are then secured in place in a slot in the capillary tube which prevents rotation of the waveguide relative to the single fiber.

A Mode Transducing Antenna

Abstract: At power levels above those readily handled by standard waveguide, and for long waveguide runs, the TE/sub 01/ mode in circular guide is a preferred transmission line approach. At the antenna, this mode is generally unsuited to radiation since it forms a conical, instead of a pencil beam. Phase and polarization shifting techniques at a reflecting surface cause the illumination to be transduced to form a pencil beam. The techniques have been demonstrated in a 33-GHz model.

Light scattering in optical waveguides.

Abstract: The coupling of scattered radiation from Raman-active core and cladding regions to the waveguide normal modes is analyzed by treating the scattered light as a classical incoherent effective source. Numerical results are presented for the limiting cases of single-mode or highly multimode excitation of the fiber. The effect of a finite frequency shift between the exciting and scattered fields is also considered. It is found that radiation produced by scattering of the evanescent field in the cladding is coupled to the guided modes with a peak efficiency of about one-fifth that of light scattered in the core. The implications of these effects for optical devices utilizing fibers are briefly discussed.

Mode coupling coefficient measurements in optical fibers.

Abstract: The measuring method of the mode coupling coefficient was developed to apply to multimode fibers. It is shown that a mode coupling coefficient can be calculated from each mode group pulse waveform. Measurements were made with two cases of long-length and short-length multimode step-index fibers. As a result, it was observed that in the fiber sheathed by nylon, there existed a strong mode coupling in lower order modes. When it is assumed that this mode coupling is generated by microbendings, the correlation length and standard deviation of the curvature were estimated as 2.0 mm and 4.2 × 10−4 mm−1, respectively.

Electro‐optic guided‐to‐radiation mode conversion in Cu‐diffused LiTaO3 waveguide with periodic electrodes

Abstract: We report electro‐optically controlled guided‐to‐radiation mode conversion at 0.63‐μm wavelength in a waveguide formed by electrodiffusing Cu ions in a c‐plate crystal of LiTaO3. The efficiency of conversion from TE1 guided mode to TM radiation modes was measured by a low‐frequency (0.5 kHz) modulation experiment with special attention paid to the dependence of the modulation efficiency on the longitudinal period of coplanar electrodes. Maximum mode conversion was observed at a 298‐μm period where the efficiency was 21%/cm (16% for actual interaction length of 7.4 mm) with an applied voltage 600 V across the 45‐μm gap. With a slight deviation (±10 μm) in the period, the efficiency was decreased to less than 10%/cm and its sensitivity is attributed to the change in the transverse field overlap of the coupled modes.

Single mode grating coupling between thin-film and fiber optical waveguides

Abstract: Optical directional coupling by means of a periodic perturbation is described between a clad, single mode, cylindrical fiber, and a thin-film planar waveguide. Codirectional coupling was achieved to a clad fiber, with 0.4 percent efficiency; allowing for the geometric mismatch, this is effectively 30 percent coupling. The fiber guided mode is accessed by reducing the fiber diameter by heating and pulling. Mode phase matching is achieved with a periodic grating sputter etched into the film. Unwanted grating induced interactions are minimized by restricting the guide parameters or by modifying the coupling mechanism. A perturbation analysis is used to calculate coupling coefficients and the design procedure for an optimum structure is explained. Measurements on grating couplers are described and the film/fiber codirectional coupling is shown to be effected by the grating.

Strip-loaded waveguide formed in a graded-index LiNbO 3 planar waveguide

Abstract: An As40Se50−xSxGe10 film strip loaded waveguide, formed in the graded-index LiNbO3 planar waveguide, has been demonstrated. Analytical results show that the optical field confinement in the waveguide loaded by the high refractive index film becomes large near the film cutoff thickness for the fundamental mode. Photostructural effect of the chalcogenide glass overcomes difficulty in precisely controlling film thickness. A 3-D waveguide has been achieved by loading As40Se10S40Ge10 film 10 μm wide on a Ti diffused LiNbO3 planar waveguide. Optical confinement in the waveguide has been improved intensively with the aid of the photostructural effect of the film.

Measurement of mode conversion coefficients and mode dependent losses in a multimode fiber

Abstract: A new method for the simultaneous measurement of the loss and mode conversion coefficient in multimode fibers is proposed, which utilizes the differences of the mean transit time among modes. The principle is applied to a step-index fiber, and the mode conversion coefficient and mode dependent loss are obtained. A procedure to minimize the experimental errors is also discussed. In the experiment the mode conversion coefficient decreases gradually when the mode angle increases and increases rather quickly when the angle approaches the critical angle of the fiber.

Optical communication line

Abstract: PURPOSE:To realize the optical communication line of a wide transmission band width substantially equal to that of a single mode fiber by using the mode filter of the simple construction. CONSTITUTION:When the value of the standardized frequency V of a linear multimode optical fiber is larger than 2.4, the effect of confinement of the basic mode into the core is large and the intensity of the basic mode at the boundary of the core and clad becomes weak. Hence, it becomes difficult to receive the effect of disturbances at the boundary of the core and clad and the losses become smaller as against various bendings occuring in cabling and laying. Hence, the single mode is substantially made possible by adding a mode filter 4 function near the receiving end of an optical fiber 5.

Mode mixing effects in optical fibers caused by sheathing and multistranding: measurements

Abstract: Mode coupling coefficients were measured in a multimode step-index fiber at different steps of the process of manufacturing a multistrand optical fiber cable. It was established that the mode coupling coefficients of the unsheathed fiber were relatively small and nearly the same for all the guided modes. By sheathing with nylon and by multistranding, the mode coupling coefficients between lower-order modes increased rapidly due to microbends, while in the vicinity of the highest-order mode, they remained unchanged. From impulse response waveforms and baseband frequency responses, it was observed that mode mixing effects became more noticeable as the mode coupling coefficients increased. Also, it was found that the excess loss caused by microbends was relatively small.

Mode losses in unstable resonators with rounded edges.

Abstract: The waveguide approach to unstable resonators is employed for determining the eigenmode losses in unstable 2-D and 3-D resonators with perfectly reflecting round-edged mirrors. It is found that edge rounding in cylindrical mirror configurations leads to mode detachment at lower Fresnel numbers than for the sharp-edged case and that the loss behavior approaches more closely the geometric optical limit for infinite mirrors. Similar effects are observed in resonators with circular mirrors, where mode detachment does not occur at all when the mirror edges are sharp. It is also shown that edge rounding improves the separation between the low-loss detached mode and the next higher order modes.

An efficient acousto‐optic TE?TM mode converter utilizing a doubly confined optical and acoustic waveguide structure

Abstract: Several types of doubly confined optical and acoustic waveguide structure for extremely efficient acousto‐optic TE?TM mode conversion are proposed and theoretically analyzed. It is shown that only 1 mW of acoustic power is required for 100% TE10?TM10 mode conversion along the x‐axis on Y‐cut LiNbO3 with an interaction length of 5 mm and a waveguide width of 30 μm.

50mm diameter TE01 mode helical waveguide optimisation

Abstract: Changes to the design of the UK Post Office long distance TE01 mode circular waveguide are described. A 250 m length of the new waveguide structure has been manufactured and assessed; its transmission performance in straight route sections has been improved and much lower loss in curved sections has been achieved.

Fiber for optical transmission

Abstract: PURPOSE:To achieve the reduction in loss at the mode converting of fibers for optical transmission by changing the distribution type of refractive indices throughthe use of of mode filters and avoiding direct coupling with a radiation mode.

Coherence and beam geometry of a superradiant dye laser

Abstract: This paper reports the observation of a well defined radiation mode emitted from a superradiant dye laser pumped by a pulsed nitrogen laser. Beam geometry and spatial coherence of the dye laser are studied in connection with the pumping geometry. It is shown that under favorable pumping conditions most of the excited molecules radiate into the same spatial mode by stimulated emission. The associated mode structure is also calculated based on a model of a properly phased dipole distribution. The calculated emission pattern reproduces the observed far field pattern closely.

Coupled-mode theory of a fiber laser-amplifier

Abstract: The behavior of a fiber laser-amplifier is considered, including the effects of mode coupling, radiation losses, and noise. In the linear regime, a competitive effect between mode coupling and losses is present, in the sense that mode coupling tends to equalize the gains of different modes, while radiation losses favor low-order modes. Thus, single-mode amplification and collective amplification are predominant, respectively, in the weak and strong coupling case. In the saturated regime a mode coupling effect is found that is independent of fiber imperfections and relies on the nonlinear nature of the process.

Mode attenuation in multimode three-layer fiber waveguides

Abstract: A theoretical analysis is made of the mode attenuation in a multimode three-layer fiber waveguide consisting of a core and two (intermediate and outer) claddings, the outer cladding being made of a polymer material. It is shown that losses can be reduced to a value close to that characteristic of the core material if the core diameter and aperture are increased and the wavelength is reduced.

Radiation losses in dielectric optical waveguides

Abstract: The effect of irregularities and deviations from the perfect structure of the ideal waveguide is to scatter some of the guided power carried by the modes of the ideal waveguide incident on the irregularity. This scattered power is redistributed over the (non-attenuating) bound modes of the structure and into the radiation modes of the waveguide. The study of this redistribution of the power over the discrete (bound) mode spectrum can be adequately analysed by conventional electromagnetic Coupled Mode Theory. However, the application of this technique for the analysis of the radiative losses, that is, the coupling into the radiation modes of the waveguide proves to be extremely tedious due to the difficulty of normalisation and orthogonalisation of these \"improper\" modes. The aim of this thesis is to present alternative techniques to the Coupled Mode Theory analysis, which provide simple treatments of these radiation loss processes in weakly guiding dielectric optical fibres. The philosophy of the presentation is to present the technique and choose the least number of practical examples that elucidate the strengths and deficiencies of each approach, rather than list ad nauseum a wide range of practical examples.

Analysis of nonresonant slots in a notched-wall dual-mode rectangular waveguide

Abstract: Some radiation characteristics of a narrow nonresonant slot close to, or straddling, the centre line of a notched-wall dual-mode waveguide are presented, and compared with the behaviour of similar slots in the corresponding single-mode guide The theoretical results have been evolved using Bethe's small-hole diffraction theory They are supported by some experimental evidence

Influence of the operation regime of an injection laser on the efficiency of coupling of its radiation into an optical waveguide

Abstract: A study was made of the efficiency of coupling of radiation emitted by heterolasers (including lasers made of new multicomponent compounds and emitting at 1.08 μ) into a multimode fiber by direct contact or by matching with a microlens at the fiber end. A coupling efficiency up to 0.9 was obtained using a strip heterolaser and an SiO2 fiber (light losses ~25 dB/Km). An increase in the pump current lowered the coupling efficiency. Experiments on a single-mode heterolaser with an external selective resonator showed that one of the reasons for the nonmonotonic dependence of the coupling efficiency on the pump current was the mode deformation as a result of self-focusing effects.