Showing papers on "Single-mode optical fiber published in 1974"

Low‐loss single‐mode optical waveguides and efficient high‐speed modulators of LiNbxTa1−xO3 on LiTaO3

Abstract: Low‐loss LiNbxTa1−xO3 optical waveguides have been made by a simple diffusion technique. They are used to demonstrate over 80% base band electro‐optic modulation with frequency capabilities extending to the microwave range. Voltages below 5 V and drive powers below 2/10 mW/MHz are required.

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.

Hyperbolic type optical fiber lens coupler for coupling the fiber to an optical line source

Abstract: The angular profiles of optical radiation emitted by certain conventional optical sources, such as a semiconductor P-N junction laser, are similar to those of optical line sources. In order to increase the optical coupling efficiency from such optical line sources to an optical fiber, the input end of the fiber is shaped essentially into the form of a hyperbolic cylindrical lens by first forming a flat wedge at this end of the fiber and then finishing the wedge to the desired essentially hyperbolic cylindrical surface. This finishing can be accomplished by either a flame (heat) type polishing or a mechanical (fine abrasive) type polishing.

Resonator Theory for Hollow Waveguide Lasers

Abstract: A numerical technique has been developed for analyzing the transverse modes of waveguide lasers with external mirrors. Propagation outside the guide is computed with the Fresnel-Kirchhoff diffraction integral and within the guide by decomposing the fields into the characteristic modes of the guide structure. The transverse modes of the entire waveguide–mirror system fall into a number of distinct classes: TE0m, TM0m, EH1m, EH2m, etc. For each class of modes, the, corresponding guide modes form a complete and orthogonal set and may be used as basis vectors to describe those modes. This reduces the mode analysis of the waveguide resonator to the diagonalization of a small (5 × 5 or 10 × 10) complex matrix. Guide losses, coupling losses, and mode shapes will be discussed for a number of interesting cases, with the Fresnel number of the waveguide ranging from 0.1 to 1.0 and with various values of mirror curvature and position. It will be shown that some values of resonator parameters are particularly advantageous for achieving single mode operation.

Optical fiber power taps employing mode coupling means

Abstract: In an optical fiber power tap, signal power is coupled out of an optical fiber waveguide by a dielectric body disposed in a coupling relationship laterally offset from an intermediate length of the fiber, and is converted to a representative electrical signal suitable for utilization by a photodetector disposed adjacent to the dielectric body. In accordance with the disclosed invention, the operation of the fiber tap is improved by combining therewith means for coupling a portion of the signal power in the fiber from lower order modes to higher order modes. The mode coupling means illustratively comprises a pair of corrugated plates which is pressed against the fiber to periodically deform a region of the fiber just preceding the fiber tap. Provided the spatial periodicity of the corrugations in the plates is properly selected, and the pressure exerted on the plates is sufficiently large, the periodic deformation of the fiber causes the desired higher order mode coupling, and assures that there is sufficient power distributed in the higher order modes of the fiber for the dielectric coupling body of the fiber tap to couple out. The mode coupling means can be employed to provide an adjustable optical fiber tap, and can facilitate the tapping of clad fiber waveguides.

Radiation fields of GaAs-(AlGa)As injection lasers

Abstract: Theoretical radiation patterns from a multilayer model of a dielectric waveguide are fitted to single mode experimental profiles of three symmetrical double-heterojunction cavities to test the adequacy of the model and study the effect of the dielectric parameters on the beam pattern. The radiation from the normal TE modes is approximated by plane waves, while for the TM modes it is given by boundary value solutions of the Maxwell equations. The adequacy of the theory is shown by the faithfulness of the fit out to large beam angles and low intensities, and the agreement of the adjusted cavity parameters to the experimental values. Small changes in either the cavity thickness or the dielectric constant of the internal n-type region have similar first order effects on the angular position of the minima in the profile as well as in the amplitude of the sidelobes. Depths of the minima decrease with departures of the structure from planarity. Pattern distortion from mode coupling at the interfaces and facets is not observed.

Star coupler for single mode fiber communication systems

Abstract: An optical coupler system in which optical communications from separate cers are transmitted over separate lines of a single optical fiber to a coupler which combines the information and transmits the combined information over different single fibers to other communication centers.

Excitation of Fundamental and Low-Order Modes of Optical Fiber Waveguides by Gaussian Beams. 1: Tilted Beams

Abstract: The optimum condition for coupling energy into an optical fiber waveguide is theoretically computed for an incident beam with a Gaussian intensity distribution and a curved wavefront. The dependence of the modal excitation efficiencies on the tilt angle of the incident beam and on the refractive index difference of core and cladding is discussed. Numerical results show that maximum efficiency for the HE11 mode can be achieved when the beam radius is chosen to be approximately 0.65 times the core radius. Excitation efficiencies of low-order modes, HE1m (m ≥ 2) approach 80% at their cutoff when the beam size is suitably larger than the core diameter.

GaAs Twin-Laser Setup to Measure Mode and Material Dispersion in Optical Fibers.

Abstract: Two synchronously self-pulsing GaAlAs lasers, emitting at slightly different wavelengths, are combined in a setup which evaluates mode and material dispersion in multimode fibers. The results show that material dispersion which causes a pulse broadening of 3.6 ns/km, which may represent a serious limit to the bit rates achievable in a multimode fiber system using an LED source.

Single longitudinal mode operation of cw junction lasers by frequency-selective optical feedback

Abstract: cw oscillation in a single longitudinal mode is reported for stripe‐geometry junction lasers operated at room temperature with frequency‐selective optical feedback. The frequency‐selective feedback, obtained by optically coupling a diffraction grating to the diode, is used to select a single mode of the normally multimode spectrum generated by the diode laser itself. Single‐frequency oscillation is observed for currents up to 30% above threshold. The oscillation frequency has a linewidth determined primarily by the internal dynamics of the diode and is tunable over the entire multimode spectrum. Additional experimental results demonstrate the homogeneous broadening of the radiative line in these lasers.

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.

Single‐mode cw dye laser with large frequency range tunability

Abstract: We describe in this letter a cw single‐mode dye laser whose mode frequency can be tuned continuously over a relatively wide range of typically 10 GHz.

Propagation characteristics of multimode fibers with graded core index

Abstract: Bending loss and impulse response of multimode fibers with graded-index core have been studied. A multimode fiber which has a large index difference is profitable because of low loss in bends. Impulse response of the fiber in which all of the possible modes are excited with the same intensity depends remarkably on the fourth-order term of the index profile. A small value of mode delay of 0.15 ns/km is estimated to be obtainable in a fiber having an optimum index profile and index difference ratio of 0.05 using incoherent light.

Pulse dispersion for single-mode operation of multimode cladded optical fibres

Abstract: Pulse dispersions as low as 0.4 ns/km have been measured in multimode cladded fibres at a normalised frequency V = 125 and for a constant bend radius of 5.5cm. Particularly when the number of launched modes is small, the pulse dispersion, as well as the polarisation and angular width of the output beam, are strong functions of the degree of mode conversion.

Fiber distributed feedback laser

Abstract: Utilizing round optical fibers as communication channels in optical communication networks presents the problem of obtaining a high efficiency coupling between the optical fiber and the laser. A laser is made an integral part of the optical fiber channel by either diffusing active material into the optical fiber or surrounding the optical fiber with the active material. Oscillation within the active medium to produce lasing action is established by grating the optical fiber so that distributed feedback occurs.

Microwave Method for Measurement of Fiber Orientation in Paper

Abstract: A microwave method for the measurement of fiber orientation in paper, is described. The method uses a dual mode microwave cavity where the modes have equal resonant frequencies and mutually perpendicular electric fields. The paper is inserted through the cavity so that the electric fields of both modes are in the plane of the paper. If the orientation of the fibers in the paper is not random the resonant frequencies will differ due to the dielectric anisotropy of the paper. This difference is measured and can be calibrated to give the amount of fiber orientation. Moisture and thickness variations in paper can cause calibration errors. However, if the variations are within certain limits, these errors can be eliminated by a suitable selection of the resonator bandwidths.

A single-mode nitrogen-pjmped tunable dye ring-laser

Abstract: This paper describes a nitrogen-pumped tunable dye ring-laser. The cavity is made with prisms. Oscillation in single transverse and longitudinal mode was easily obtained.

Birefringent coupler for integrated optics.

Abstract: An optical coupler has been developed whereby a birefringent prism or substrate is utilized to couple light into thin film optical waveguides. The coupler is designed so that the incident light, polarized as an extraordinary wave, passes through the birefringent material at such an angle that the index of refraction in the input medium is higher than that of the waveguide. In addition, the orientation of the birefringent material is chosen so that the reflected waves within the waveguide are incident on the input medium at such an angle that the medium index of refraction appears lower than that of the waveguide. In this way, the light is trapped within the waveguide and, when the light is incident at the appropriate angle, this light propagates in the waveguide as a single mode. Coupling takes place over the whole area common to the waveguide and input medium; thus, neither a sharp edge nor a trapped evanescent region are required to prevent the light coupled into the waveguide from leaking back into the coupling prism. The absence of a boundary makes it practical to couple wide beams of light into planar guides. Couplers made of calcite have been used to couple TM He–Ne laser light into multimode waveguides formed of liquids and optical cement.

Optimal trade-off of mode-mixing optical filtering and index difference in digital fiber optic communication systems

Abstract: In a digital fiber optical communication system, the optical power required at the receiver input to achieve a desired error rate depends upon the shape of the received pulses. In systems employing multimode fibers and/or broadband sources, we can experience pulse spreading in propagation because of the group velocity differences of different modes or because of dispersion. In an effort to control or compensate for pulse spreading, we can trade off coupling efficiency between the light source and the fiber (by varying the core-cladding index difference or bandlimiting the source), scattering loss in the fiber (by introducing mode coupling), and equali sation in the receiver at baseband. This paper investigates the optimal trade-off for various fiber-source combinations.

Single mode operation of a tunable cw dye laser

Abstract: Details of single longitudinal mode operation of an argon laser pumped cw dye laser using a low-loss interferometric mode selector are described. Typical output powers of 5 mw with a bandwidth of 30 MHz for an input power of 700 mw have been obtained.

OPTICAL GUIDING AND ELECTRO-OPTICMODULATION IN Ga As EPITAXIAL LAYERS*

Abstract: Single mode TE or TM propagation is demonstrated in an optical waveguide consisting of a high resistivity semiconductor (GaAs) layer (τ= 10 μ) which is sandwiched between a metal film and a lower resistivity semiconductor. A reverse bias applied to the metal-semiconductor Schottky barrier causes an electro-optic retardation (or, in general, phase variation) which can be used for modulation purposes. Amplitude modulation with a “half-voltage” V1/2 = 84 volts is demonstrated at γo 1.15 μ with a sample 2.4 mm long.