Showing papers on "Single-mode optical fiber published in 1978"

cw three-wave mixing in single-mode optical fibers

Abstract: Strong continuous three‐wave mixing of 514.5‐nm argon laser light in a single‐mode fiber is reported. The effect, due to the third‐order nonlinearity of silica, has been observed for light whose frequency spectrum consists of either a few discrete monochromatic frequency components separated by ∼1 GHz or a quasicontinuous distribution of frequencies having a spectral envelope ∼4 GHz wide. We show that the effect provides a simple and effective method for measuring the nonlinearity of silica. In the first manifestation of the effect, the nonlinearity mixes the frequency components to produce new frequencies. In the second, multiple mixing occurs that broadens the quasicontinuous spectrum. This manifestation of the effect is large; broadening by a factor of 4 has been observed with lower intensity levels than are required to produce stimulated Brillouin scattering in the same fiber. A theoretical model is presented to describe spectral broadening by three‐wave mixing for the case of small broadening. The effect of three‐wave mixing on the operation of continuous stimulated Brillouin and Raman oscillators is also discussed. Finally, it is noted that the presence of this effect may constrain the design of long‐haul single‐mode fiber optical communication trunks.

Polarization and magnetooptic properties of single-mode optical fiber

Abstract: The polarization and magnetooptic properties of a sample of single-mode optical fiber have been investigated. The fiber acts as a linear retarder, and the degree of retardation is dependent on the external pressure applied to the fiber. The stress optic coefficient is found to be 8.72 x 10(-10) N(-1) m(2). The direction of linear polarization is rotated when a longitudinal magnetic field is applied to the fiber (Faraday effect). The Verdet constant is 1.56 x 10(-2) min A(-1). The intrinsic specific linear retardation of the fiber is found to be less than 0.44 rad m(-1) from the magnetooptic measurements. The fiber has been used in an experimental current measurement device.

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

Interferometer apparatus using electro-optic material with feedback

Abstract: A nonlinear interferometer apparatus is disclosed wherein a Fabry-Perot cavity is substantially filled with an electro-optic material having terminals to which a potential can be applied. A single frequency, single mode, linearly polarized source of optical radiation is coupled into the cavity and a beam splitter at the output of the cavity deflects a portion of the optical energy coming from the cavity. This deflected portion of output optical energy is incident on a photodetector whose output is amplified and connected to the terminals of the electro-optic material. The resulting device exhibits differential gain, and can function as an optical switch, limiter or optical memory element. With sufficient amplification of the detector output, an optical quantizer is provided. By properly adjusting the cavity length and providing a substantially constant optical signal to the input end of the cavity, an amplified replica of an amplitude modulated optical signal applied to the photodetector appears at the output end of the cavity.

Polarization characteristics of noncircular core single-mode fibers

Abstract: It is well known that geometrical or dielectric imperfections in conventional graded-index single-mode fibers depolarize light after a few centimeters. A slight improvement in the polarization performance of these fibers is achieved by introducing noncircularity in the core shape. This is evident from the measurements on borosilicate fibers with dumbbell shaped cores. This result is correlated with Marcatili’s analysis, which shows that changing the core geometry, from square to rectangular, does not appreciably alter the difference in the propagation constants of the two fundamental modes with orthogonal polarizations. Thus, the noncircular geometry and the associated increase in stress-induced birefringence introduced during the manufacturing process alone are not sufficient to improve the polarization performance, and the enhancement of the anisotropic birefringence is necessary to achieve single-polarization fibers.

Fusion splices for single-mode optical fibers

Abstract: A practical low loss splicing method based on the discharge fusion for single-mode fibers was developed. Average splice losses of 0.4, 0.2, and 0.1 dB for fibers with 5.2, 7, and 10 μm core diameters, respectively, are obtained by a simple apparatus utilizing the self-alignment effect due to the surface tension of melted fiber ends. The surface tension effect is analyzed both experimentally and theoretically. Experimental splice losses, both after and during heating, coincide with the theoretical estimated values. It was found that the optimum heating temperature for low loss splices is near 2000°C at 8.5 W electric discharge power. Splicing loss causes are examined. The main cause of the practical splice loss is the residual core axis misalignment caused by an insufficient surface tension effect and core eccentricity with respect to cladding.

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.

Self-coupled phenomena of semiconductor lasers caused by an optical fiber

Abstract: Output intensity noise spectra of semiconductor lasers coupled to optical fibers with end-butt joints are measured to show periodic peaks with a fundamental frequency decided only by the fiber length. The frequency is found to coincide with the reciprocal of the roundtrip time of the light beam in the optical fibers.

Broadening mechanism in semiconductor (GaAs) lasers: Limitations to single mode power emission

Abstract: A nonlinear steady-state theory of the emission spectrum of semiconductor (GaAs) lasers above threshold is developed, and limitation to power in a single longitudinal mode is studied. The nonlinear steady-state rate equations describing the power and the gain are solved iteratively. The model is based on the well known idea that the gain always saturates somewhere below the loss, and power sharing among the modes is dependent on the relative gain of the modes with respect to the loss level. The limitation to single mode power is essentially due to the uneven rate of saturation of the gain of the different modes as they approach the loss level asymptotically, with the dominant mode having the fastest saturation. The rate of saturation of the gain of different modes depends on the power emission spectrum and the intraband relaxation rate of the carriers. In this work, the relaxation is accounted for by using the generalized spectral weight function to describe the carriers. The dependence of maximum single mode power on intraband relaxation time is obtained. It ranges from a few milliwatts for relaxation time of the order of 10-12s to hundreds of milliwatts for relaxation time of 10-13s. The predictions of the model on gain saturation spectra, and carrier lifetime spectra agree well with experimental observations. The gain is seen to saturate near the lasing energy but continues to increase at a reduced rate at higher energy levels. The carrier radiative lifetime is found to decrease sharply in the vicinity of the lasing mode energies.

Bending losses of coated single-mode optical fibers

Abstract: Peaks appear in bending losses of coated single-mode fibers due to interference between the guided mode and rays which are radiated from the guided mode and are reflected at cladding-coating boundary. This paper reports derivation of bending loss formulas for coated slab waveguides and coated fibers. Plane wave concepts are also used to explain the appearance of the loss peaks. Measurements were performed by using two coated single-mode fibers. The agreement between theory and experimental results is found to be excellent. It is possible to obtain the refractive index difference from measured peak wavelengths.

Propagation in radially-inhomogeneous single-mode fibre

Abstract: The propagation of electromagnetic waves along a radially-inhomogeneous single-mode optical fibre with small index difference between core and cladding is studied. As in the case of the step-index fibre the propagation modes are weakly guided and their transverse fields are essentially polarized in one direction. The simple field descriptions and characteristic equation obtained here enable mode spot size and bending loss to be determined and are useful for practical design work of radially-inhomogeneous single-mode optical fibres.

Measurements of the frequency dependence of normal modes

Abstract: An experiment using a vertical array to detect acoustic normal modes in shallow water is described. A high signal‐to‐noise ratio was achieved by the use of pseudorandom pulse sequences to modulate the projector. Wide bandwidth signals and a tunable acoustic source enabled the frequency dependence of normal modes to be measured and results are in good agreement with the theory. An improved method of extracting the signal present in a single mode is described and used to examine pulse shapes and frequency spectra of individual modes.

Polarization effects in short length, single mode fibers

Abstract: The ability to maintain linearly polarized output in single mode fibers is essential for utilization of polarization dependent receiver circuitry Our measurements with long lengths of fiber (200 m) indicate that we can find input polarization angles which yield essentially linearly polarized output However, we found that these polarization effects are greatly influenced by the presence of physical stress on the fiber such as stress due to bending, twisting, mounting, and other variations in ambient conditions We conducted several experiments on short length fibers where special precautions were adopted to assure repeatability of the measurements Our results indicate the existence of a general theoretical model that predicts the output polarization characteristics as a function of input polarization and fiber length The model assumes the presence of two asynchronous, orthogonal modes, uniformly coupled over the entire fiber length The model, however, cannot distinguish between uniformly coupled and uncoupled mode cases based on the output radiation measurements

Low-birefringence single-mode optical fibers: preparation and polarization characteristics.

Abstract: A number of applications require single-mode optical fi­ bers in which the state of polarization of the guided light is sustained. The solid-core optical fibers described in the literature generally exhibit strong birefringence and also usually have a depolarizing effect. In this Letter we describe experiments on single-mode fi­ bers that were prepared under carefully controlled conditions and studied for their polarization behavior. The most fa­ vorable results were noted for fibers prepared by chemical vapor deposition (CVD). First, a Si02-B2O3 glass was de­ posited from the gas phase as cladding on the inside wall of a fused-silica tube, followed by pure quartz glass as the core material. The tube was then collapsed into a rod by raising the temperature and thereafter drawn into a fiber. Directly after leaving the furnace, the fiber was coated with silicon resin. Through precise control of the various step of the process, especially during the collapsing of the tube, a circular cross section in preform and fiber were realized. Figure 1 shows a transmission micrograph of a cross section of the preform. A comparison of round and oval fibers made of glass of the same composition showed a circularly symmetrical cross

Single-mode collapse in 6328-A He--Ne lasers

Abstract: Internal-mirror 6328-A He-Ne lasers have an intramode frequency difference when placed in a transverse magnetic field. When the magnetic field strength is increased until the Zeeman frequency shift approaches the mode spacing, mode coupling is nduced. In some multimode lasers, this mode coupling results in a collapse into single-mode operation with no loss of over-all laser power in the output. The intramode frequency difference can be exploited to produce a simple frequency stabilization system whose frequency variation is less than 1 part in 10(10). Plots of beat frequency variation and single-mode tuning range are included.

High-speed optical pulse transmission at 1.29-µm wavelength using low-loss single-mode fibers

Abstract: Optical-fiber transmission experiments in the 1.3-μm wavelength region are reported. GaInAsP/InP double-heterostructure semiconductor laser emitting at 1.293 μm is modulated directly in nonreturn-to-zero (NRZ) codes at digit rates tanging from 100 Mbit/s to 1.2 Gbit/s. Its output is transmitted through low-loss GeO 2 -doped single-mode silica fibers in 11-km lengths. Transmitted optical signals are detected by a high-speed Ge avalanche photodiode. Overall loss of the 11-km optical fibers, including 11 splices, is 15.5 dB at 1.3 μm. Average received optical power levels necessary for 10-9error rate are -39.9 dBm at 100 Mbit/s and -29.1 dBm at 1.2 Gbit/s. In the present system configuration, the repeater spacing is limited by loss rather than dispersion. It seems feasible that a more than 30 km repeater spacing at 100 Mbit/s and a more than 20 km even at 1.2 Gbit/s can be realized with low-loss silica fiber cables, whose loss is less than 1 dB/km. Distinctive features and problems associated with this experimental system and constituent devices are discussed.

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.

Multiple fiber end fire coupling with single‐mode channel waveguides

Abstract: We demonstrate a practical four‐port flip chip end fire coupler useful for providing efficient coupling between single‐mode fibers and Ti‐diffused LiNbO3 channel waveguides. Channel waveguide ends were prepared for coupling by cleavage. With a configuration suitable for multiple fiber coupling, coupling efficiencies up to 47% have been measured for TM polarization. Using micropositioners to align single input and output fibers with respect to the channel waveguide, end fire coupling efficiency was 58% (TM). The effect of waveguide mode mismatch and misalignment on coupling efficiency is investigated. We also observe double refraction and leaky mode propagation with TE polarized input to LiNbO3 channel waveguides running perpendicular to the cleavage plane. With suitably prepared channel waveguides on LiTaO3, these effects are small or absent.

Method of fabricating an elliptical core single mode fiber

Abstract: A method of fabricating a low loss single mode optical fiber having an elliptically shaped core comprising subjecting a multi-layered coated substrate having at least a barrier, cladding and core layer to partial collapse, first on one side and then on the other side, to produce an intermediate product having a substantially cross-sectional elliptical shape of at least the core layer. This is followed by collapsing the composite structure into a cylindrical shaped optical fiber preform having an elliptically shaped core portion which is then heated and drawn into a single mode optical fiber having an elliptically shaped core portion.

Optical gyroscope system

Abstract: An optical gyroscope is described of the type which passes light beams in opposite directions through a single mode fiber optic wave guide that extends in a circle or coil, and which enables measurement of rotation rate of the coil by measuring the relative phase shifts of the beams by interferometric techniques, wherein simplification and enhanced accuracy are obtained. Beam splitting and phase shifting of the light is facilitated by utilizing brief pulses of light and by using light-controlling devices which are operated for a brief time only when the light pulse passes in one direction through the device but not at a different time when the pulse is passing in the opposite direction through the device. High accuracy in rotation measurement is achieved at both very slow and very fast rotation rates, by alternately operating the system so that at zero rotation the interfering waves are alternately 90° out of phase and in phase. Linear polarization of the light beams is maintained by coiling the full length of the optic fiber in a single plane so that bending stresses induce a birefringence in the waveguide, thus altering the propagation constants of the two polarized modes and decoupling them.

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.

Laser-fiber optic interferometric strain gauge

Abstract: This invention is directed to a laser-fiber optic interferometric strain gauge. This is a strain gauge that can be read out by means of fiber optics, which has no power other than optical at the strain gauge. Laser light is directed into two single mode fibers, which fibers are attached to a supporting member the strain of which is to be measured. The ends of the fibers are brought into close proximity and the light coming out of the two fibers interferes. When the supporting member is strained the fringes move giving a measure of the strain.

Truncated Parabolic-Index Fiber with Minimum Mode Dispersion

Abstract: The use of a parabolic-index fiber as an optical transmission line has been receiving extensive attention because of its excellent mode dispersion characteristics. In the present paper, the modal dispersion in the optical fiber with truncated parabolic index distribution is analyzed theoretically in detail by using a variational method. Taking the influence of the cladding upon the propagating modes into consideration, it is found that there exists an optimum index distribution for which the modal dispersion is minimized. The standard deviation of the normalized group delay of propagating modes is used to estimate the modal dispersion behavior of the fiber.

Method for fabricating optical fibers with enhanced mode coupling

Abstract: A method of fabricating an optical fiber having an asymmetric refractive index profile that varies periodically along the length of the fiber is described. The prescribed variations are obtained by means of a fabrication process that includes periodically varying the source of at least one of the fiber materials in a direction transverse to the fiber-drawing direction. This transverse periodicity is translated into the desired longitudinal periodicity along the fiber by the fiber-drawing process.

Single-mode-fiber refractive-index profile measurement by reflection method

Abstract: A new technique for measuring the refractive index profile of single-mode fibers based on the reflection method is described, and experimental results are demonstrated. When the core radius of a test fiber is only a few times larger than the laser beam spot size, the reflected power distribution does not indicate the refractive index correctly. However, the true index profile can be calculated from the beam spot size and the reflected power distribution with high accuracy: 0.3-microm spatial resolution and 5% relative refractive-index resolution are obtained for practical single-mode fibers.

Production of optical fiber

Abstract: PURPOSE:To prepare inexpensively optical fiber having low transmittance loss, by depositing a glass layer having a given refractive index on an inner wall face of anhydrous quartz tube as a starting material, collapsing it under heating, inserting it a quartz tube for adjustment, drawing them in this state under heating, spinning them. CONSTITUTION:A glass layer having a reractive index equal to a quartz tube is deposited on the inner wall face of the anhydrous high-purity quartz tube (e.g., inner and outer diameter are about 13 and 15mm., respectively) by chemical vapor deposition method, and a core glass layer having a higher refractive index is further deposited on it. The reaction tube is then collapsed to form the parent material 3 for optical fiber consisting of the glass layers 1 and 2(e.g., D2 7.5mm., D1 1.5mm.). The parent material 3 is inserted into the quanrtz tube 4 for adjustment, they are drawn in this state, spun in an integrated manner, to give the optical fiber 5(e.g., d2 9mum, d1 125mum) having an adjusted ratio of a core diameter to an outer diameter. Consequently, single mode optical fiber having low OH group absorption loss can be prepared in a short time at low cost.

Dual beam double cavity heterostructure laser with branching output waveguides

Abstract: A dual beam laser incorporates a tapered layer within the optical cavity of a GaAs-AlGaAs double heterostructure laser. Dual output waveguides are therefore provided at one of the mirrors. By suitable variations of layer thickness and/or composition, the device operates either as a tapered power divider (TPD), branching the incident power of a single mode into the two output arms of the device, or as a tapered mode splitter (TMS), diverting different transverse modes into different arms. Because the different modes of the TMS have different thresholds, the laser can be pulsed with current pulses of different amplitude to control at which of the waveguides an output appears. The TPD, on the other hand, has coupled cavities which tend to suppress all but one dominant longitudinal mode. Also described is an integrated optical circuit embodiment of the TPD in which the resonator is formed by etched mirrors.

Low-loss single-mode fibers with different B(2)O(3)-SiO(2) compositions.

Abstract: Design considerations, fabrication, and transmission characteristics of single-mode optical fibers made with different borosilicate compositions are discussed. Fibers with core sizes as large as 15 microm and refractive-index differences below 0.1% were made with the modified chemical vapor deposition (MCVD) process. Minimum losses achieved with fibers up to 1-km lengths were 1.3-1.4 dB/km in the 1.02-1.1-microm range. Microbending losses were avoided by jacketing the fibers with silicone resin. Single-mode operation is maintained in moderately overmoded fibers (V ~ 3.0) because of leakage loss through a finite cladding thickness and because of bends in the fiber axis.

Splice loss evaluation for optical fibers with arbitrary-index profile.

Abstract: A method is presented for calculating offset and tilt losses for fiber splices with axially symmetric arbitrary-index profiles by approximating the profile with a staircase function. This method is applied to a large-core dual-mode fiber with zero intermodal dispersion as well as to single-mode fibers with step- and parabolic-index profiles. When a splice loss of 0.2 dB is permitted, the normalized offset misalignment is found to be D(N) = 0.635 for the dual-mode fiber at normalized frequency v = 4.605 and a power-law exponent alpha = 4.5. The D(N) value compares favorably with the values 0.560 and 0.614 for conventional step- and parabolic-index single-mode fibers, respectively. The dual-mode fiber is superior to the step- and parabolic-index fibers with respect to permissible splice offset tolerances.