Showing papers on "Radiation mode published in 1974"

Leaky-Ray Theory of Optical Waveguides of Circular Cross Section

Abstract: The methods and most important results required to study light transmission within optical waveguides are given. Only those techniques that are simple and that lead to concise analytic expressions are presented. Special emphasis is given to the importance of leaky modes on the fiber of circular cross section and to the use of rays for the analysis of multimode fibers. It is shown thatunlike the slab, the radiation field within the circular fiber propagates for enormous distances. Both coherent and incoherent illumination are treated.Radiation loss and mode conversion due to scattering from irregularities are found by a method much simpler than coupled mode theory. A generalized Fresnel's law is presented to account for radiation loss due to boundary curvature. This loss is shown to be caused by electromagnetic tunnelling.

Mode dispersion in uniaxial optical waveguides

Abstract: Waveguide mode propagation in optical waveguides containing uniaxial media is investigated. When the optical axis of each uniaxial medium lies in the plane containing the waveguide normal and the propagation direction, waveguide propagation is described by uncoupled TE and TM modes. The phenomenon of wave-front tilt is discussed and exact mode dispersion equations are derived. The experimentally convenient situation of the optic axis lying in the film plane is also analyzed. Here mode coupling between TE and TM polarizations occurs and the propagating modes are hybrid. The wave-vector components in the uniaxial media transverse to the waveguide are shown to have ordinary or extraordinary characteristics. Exact mode-dispersion equations for the hybrid modes are derived and are shown to differ substantially from the TE and TM mode approximations when the latter are near degeneracy.

Reduction of dispersion in a multimode fiber waveguide with core index fluctuations

Abstract: Step index, multimode fiber waveguides are designed according to the disclosed invention with intentional fluctuations of the refractive index of the fiber core, with the aim of reducing dispersion by deliberately enhancing mode coupling in the fiber and, at the same time, minimizing loss in the fiber due to coupling to radiation modes. It is shown that, to avoid radiation losses, conditions should be imposed not only on the axial (z) dependence of the core fluctuations, but also on the azimuthal ( phi ) and radial (r) dependence of the core fluctuations. Specifically, a selection rule is imposed on the coupling in the fiber by providing the core index fluctuations with an azimuthal dependence governed by a function of the form cos p phi , where p is an integer equal to or greater than one. Coupling for a mode of an azimuthal mode number Nu is thereby limited by the selection rule Delta Nu p. Additionally, by limiting the location of the index fluctuations to a region below a certain maximum radius rmax in the core, coupling to modes with large azimuthal mode numbers Nu is avoided, and power loss via coupling to radiation modes is held to a minimum.

Observation of externally controlled mode coupling in optical waveguides

Abstract: Externally controlled mode coupling was observed in a 0.7-km optical waveguide fiber resulting in a significant pulse narrowing. The method of mode coupling generation requires that care be exercised in any mode coupling measurement. Observations on a 3-km waveguide failed to show evidence of mode coupling, allowing the possibility of practical external control.

Efficient acousto-optic TE⇄TM mode conversion in ZnO films

Abstract: The TE⇄TM mode conversion by acoustic surface waves in a ZnO‐film optical waveguide fabricated on a fused‐quartz substrate is described. An efficient mode conversion as high as 88% has been accomplished in a waveguide with a film thickness of 5 μm at an acoustic frequency of 226 MHz and an acoustic power of 350 mW.

Observation of noncritically phase‐matched second‐harmonic generation in an optical waveguide

Abstract: Experimental observation of noncritically phase‐matched second‐harmonic generation in a liquid/TiO2/quartz waveguide is reported. The phase‐match condition, approximately independent of waveguide thickness, was achieved by tuning the liquid refractive index. Nonlinear interaction occurs in the quartz between the fundamental (1.06 μm) TE0 mode and the harmonic (0.53 μm) TM0 mode. Mode effective indices were measured versus liquid index to determine the phase‐match point. Coherence lengths greater than 3.5 mm were achieved, but propagation lengths were limited to 4 mm by losses due to surface scattering. These losses arise from waveguide design requirements of noncritical phase matching, not experimental film quality. Conversion efficiency was approximately 0.01%.

Mode conversion in planar dielectric separating waveguides

Abstract: Separating optical waveguides are expected to become important in integrated optics. They occur whenever a branching waveguide is used (Fig. 1a) or whenever two parallel waveguides in close proximity are separated (Fig. 1b). These situations will arise in virtually all applications of electrooptic switches and waveguide couplers that depend on evanescent coupling. Using quasi-normal modes and a step transition model we have studied theoretically the mode conversion behavior of separating planar dielectric waveguides as a function of taper slope and mode synchronism, which is adjusted by branch asymmetry. When the taper slope is large in a near symmetric structure, considerable mode conversion will occur and the structure will act as a power divider (Fig. 2b). Incident power concentrated in the upper and lower parts of the structure will end up in the upper and lower arms of the branch, respectively. This behavior is usually required for conventional evanescent couplers. However, in a more asymmetric structure with smaller taper slope, mode conversion is negligible and the structure will act as a mode splitter (Fig. 2a). Mode power is then transferred to one arm of the branch or the other. The transition boundary between a power divider and mode splitter (Fig. 3) is conveniently described approximately by: equation Here Δβ is the difference in mode propagation constant for large waveguide separation, θ is the taper slope, and γ is the decay constant of the field in the separating region. This result will allow quantitative design of mode splitters or power dividers, as desired, and should aid in the design of separating waveguides with minimum radiation losses.

Transverse coupling in fiber optics part II: Coupling to mode sinks

Abstract: The number of modes that can propagate without radiation loss in oversized waveguides is sharply reduced if the waveguide is coupled to a structure supporting radiation modes, the loss mechanism being analogous to Cerenkov radiation. The coupling formula derived in Part I1 is used to evaluate the loss for a specific configuration: a reactive surface (e.g., a thin dielectric slab) acting as a waveguide, coupled to a semi-infinite dielectric acting as a mode sink. The method consists in first assuming that the substrate is finite in size and lossy and adding the losses associated with each substrate mode. The substrate dimensions are subsequently made infinite and the dissipation loss is made to vanish. The expression obtained for the radiation loss coincides with an expression obtained by solving the boundary value problem. The method is then applied to the problem of mode selection for dielectric rods coupled to dielectric slabs, which is of particular importance for optical communications and integrated optics. A 2-dB/m radiation loss is calculated for the first higher order mode when the rod radius is 10 µm, λ = 1 µm, n = 1.41, and the rod-to-slab spacing is 0.15 µm.

A partially metal-clad-dielectric-slab waveguide for integrated optics

Abstract: Characteristics of a partially metal-clad-dielectric-slab waveguide are described. In this waveguide a propagating field is confined two-dimensionally by the difference of effective indices of refraction between a dielectric-clad region and a metal-clad region. Experimental results are also described.

Theory of the single-material fiber

Abstract: The term “single-material fiber” describes a dielectric optical waveguide made of only one type of glass. The theory of this waveguide is simplified by placing the structure between two perfectly conducting planes that have very little influence on the properties of the low-order modes. The field distribution and propagation constant of the lowest-order mode are investigated and compared to an approximate theory.

Partially cladded triangular-cored glass optical fibers and lasers*

Abstract: Optical fibers that possess the characteristics of both conventional cladded fibers and of a new type of fiber have been drawn and evaluated The core has an equilateral triangular cross section with slightly bulging curved sides The rounded vertices are in contact with the inside of the cladding tube of lower refractive index Most of ther periphery of the core is surrounded by air The cavity-like waveguide modes of this new type of fiber can be described as due to skew rays being reflected systematically from the glass–to–air interfaces as they propagate along the fiber Near-field and far-field radiation patterns have been photographed and are presented A new type of laser based on these cavity-like waveguide modes is proposed, in which the triangular core is made of a material (eg, neodymium-doped glass, YAG, etc) that can be optically pumped The normal mode of operation of the laser consists of six diverging, spatially separated output beams; it can be optically pumped axially from both ends simultaneously, while being pumped from the sides

Polishing technique for optical waveguide terminations

Abstract: A technique is described which produces a good optical finish on the end of an optical waveguide at any required slope, without permanently embedding the waveguide in a holding medium.

o es and Related Radiation Losses in Optical Dielectric VVaveguides with External Higher Index Surroundings

Abstract: Absfract—Mode filter actions are found theoretically in an optical dielectric waveguide consisting of a core and a thin cladding layer which is further surrounded by an external higher index region. The propagating waves, which are usually considered to be cutoff modes, can be guided with a small amount of loss under a certain condition. These waves are defined here as quasi-guided modes. These modes tend to the guided modes of the guide when the cladding thickness increases infinitely. A method is given to estimate the losses. As an example, the radiation losses are formulated for a symmetric slab waveguide, and are found to be approximately proportional to the cube of the mode number of the quasi-guided mode. Therefore, losses of the quasi-guided modes depend strongly on the mode number. It is suggested that fibers with large core dkuneters can be used as quasi-single mode fibers by covering the clad-type multimode fibers with external higher index surroundings and choosing the parameters properly.