Showing papers on "Single-mode optical fiber published in 1996"

All-optical wavelength conversion by semiconductor optical amplifiers

Abstract: Following a brief introduction to the applications for wavelength conversion and the different available conversion techniques, the paper gives an in depth analysis of cross gain and cross phase wavelength conversion in semiconductor optical amplifiers. The influence of saturation filtering on the bandwidth of the converters is explained and conditions for conversion at 20 Gb/s or more are identified. The cross gain modulation scheme shows extinction ratio degradation for conversion to longer wavelengths. This can be overcome using cross phase modulation in semiconductor optical amplifiers that are integrated into interferometric structures. The first results for monolithic integrated interferometric wavelength converters are reviewed, and the quality of the converted signals is demonstrated by transmission of 10 Gb/s converted signals over 60 km of nondispersion shifted single mode fiber.

Solitons in optical communications

Abstract: The history of the proposal that solitons be used for optical fiber communications and of the technical developments toward making soliton transmission practical is reviewed. The causes of bit errors in long-distance soliton transmission are presented and the methods for reducing them are described. A perturbation theory suited for soliton analysis is developed. Current status and future prospects of long-distance repeaterless fiber communications are stated.

Chromatic dispersion in fiber-optic microwave and millimeter-wave links

Abstract: The influence of chromatic fiber-dispersion on the transmission distance of fiber-optic microwave and millimeter-wave links is analyzed and discussed in this paper. It is shown that dispersion significantly limits the transmission distance in intensity modulated direct detection links operating in the above 20 GHz frequency region by inducing a carrier to noise penalty on the transmitted signal. At 60 GHz, a 1 dB penalty is induced after less than 500 m transmission over standard single-mode fiber with a dispersion of 17 ps/km/spl middot/nm and the signal is completely extinct after 1 km. In remote heterodyne detection links, the dispersion induces both a carrier to noise penalty and a phase noise increase on the transmitted signal. It is shown, however, that the induced carrier to noise penalty is insignificant. At 60 GHz, the induced penalty is less than 0.3 dB after 100 km transmission. The phase noise increase proves more dominant. At 60 GHz, a 150 Mbit/s QPSK signal is limited to around 10 km of transmission.

Polarization mode dispersion, decorrelation, and diffusion in optical fibers with randomly varying birefringence

Abstract: Polarization mode dispersion and the polarization decorrelation and diffusion lengths are calculated in fibers with randomly varying birefringence. Two different physical models in which the birefringence orientation varies arbitrarily are studied and are shown to yield nearly identical results. These models are appropriate for communication fibers. We show that both the length scales for polarization mode dispersion and polarization decorrelation measured with respect to the local axes of birefringence are equal to the fiber autocorrelation length. We also show that the coupled nonlinear Schrodinger equation which describes wave evolution over long length along a communication fiber can be reduced to the Manakov equation. The appropriate averaging length for the linear polarization mode dispersion is just the fiber autocorrelation length but the appropriate averaging length for the nonlinear terms is the diffusion length in the azimuthal direction along the Poincare sphere which can be different, The implications for the nonlinear evolution are discussed.

The acousto-optic effect in single-mode fiber tapers and couplers

Abstract: All-fiber acousto-optic devices based on the null fused taper coupler have been successfully demonstrated as frequency shifters, variable splitters, switches and tunable filters. In this paper, the interaction upon which these devices are based has been extensively analyzed under a set of approximations that are valid in most cases. Simple analytical expressions for the important properties are derived, which provide a set of design rules for such devices.

A novel design of a dispersion compensating fiber

Abstract: We propose a novel dispersion compensating fiber design consisting of two highly asymmetric concentric cores. We show that the fundamental mode of the proposed fiber can have very large negative dispersion values [/spl sim/-5100 ps/(nm.km)] with larger mode field diameter (/spl sim/8-9 /spl mu/m) relative to the existing dispersion compensating fibers.

Laser-fabricated low-loss single-mode raised-rib waveguiding devices in polymers

Abstract: Organic polymeric materials offer a versatile medium for the creation of low-cost large-area optical guided-wave structures. In this work, we report on the use of a novel maskless laser-based microfabrication technique for the photochemical delineation of raised-rib single-mode waveguiding devices in polymers. This technology relies on accurate control of small refractive index differences (which is achieved by using intermiscible acrylate monomers), use of high-contrast photochemical response, as well as precise control of laser writing parameters. The devices reported here have cross-sectional dimensions and numerical apertures that match single-mode glass optical fibers. They exhibit very low losses of 0.03 dB/cm at 840 nm and exceptional thermal stability. We present the operational characteristics of bends, Y-branches, and directional couplers fabricated using this technology and compare these characteristics with those predicted from theory.

Pulse compression by nonlinear pulse evolution with reduced optical wave breaking in erbium-doped fiber amplifiers

Abstract: Nonlinear pulse evolution is studied for a fiber with normal dispersion (ND) and gain. Numerical simulations show that under certain conditions the pulse evolves into a parabolic shape, which has been shown to reduce optical wave breaking. Much as with the square pulse that forms in passive fibers with ND, the interplay of ND and self-phase modulation creates a highly linear chirp, which can be efficiently compressed. Application to an amplifying fiber/grating (prism) pair pulse compressor is considered, with an experimental demonstration of compression from 350 to 77 fs at a gain of 18 dB in an erbium-doped fiber amplifier.

Energy-scaling characteristics of solitons in strongly dispersion-managed fibers.

Abstract: We present an empirical scaling law that models the increased energy required for launching a soliton into an optical system with sections of both normal and anomalous dispersion fiber. It is shown that the inclusion of periodic attenuation and amplification can be handled as separate problems, provided that the interval between optical amplifiers is substantially different from the period of the dispersion map. These concepts are illustrated by reference to an example system comprising dispersion-shifted fiber combined with anomalous standard fiber.

Evanescent-wave guiding of atoms in hollow optical fibers.

Abstract: An atom placed in a near-resonant laser field is either attracted to or repelled from regions of high intensity depending on the sign of the laser’s detuning from atomic resonance. We have demonstrated that the optical forces induced by laser light guided in a fiber may be used to reflect atoms from the inner wall of a hollow-core optical fiber in a recent work [1,2]. In that demonstration, light was coupled to the lowest-order grazing incidence mode [3] and the laser frequency was tuned to the red side, so that atoms were attracted to the high-intensity region at the center of the fiber. Atoms guided in this way undergo a series of lossless oscillations in the transverse plane and unconstrained motion along the axis. Atoms can also be guided by the evanescent light field of the glass surface surrounding a hollow fiber. With a detuning on the blue side of resonance, atoms are expelled from the high-intensity-field region near the fiber wall. The intensity in the evanescent field is significant at a distance of .=X/2.rr into the hollow region. Consequently, the atoms are nearly specularly reflected from the potential walls. Atom propagation through the fiber in this case is similar to the propagation of light in a multimode, step-index fiber. Evanescent guiding has several advantages over guiding by grazing incidence modes: heating of the atoms due to spontaneous scattering of photons is small in the evanescent case because the atoms spend most of the time in a dark region away from the high laser intensity at the wall. In the grazing incidence configuration, atoms are guided in the high-intensity region, and consequently the spontaneous scattering rate is relatively high. Furthermore, in evanescentwave guiding, the optical potential is generated by light traveling in lossless guided modes. Small-diameter atomic guides of very long length may be practical. By contrast, grazing incidence optical modes decay exponentially with distance [3], effectively limiting the guiding distance to a

Broadband fiber-optical parametric amplifiers and wavelength converters with low-ripple Chebyshev gain spectra.

Abstract: Theory predicts that a two-pump fiber optical parametric amplifier or wavelength converter operated near the fiber zero-dispersion wavelength can exhibit a gain spectrum approximated by a Chebyshev polynomial of order 8. Under realistic conditions of pump spacing and fiber dispersion, very low-gain ripple can be obtained over a large bandwidth. For example, a dispersion-shifted fiber can provide a signal amplifier with a gain of 20 dB with 0.2-dB uniformity over a 45-nm bandwidth. Potential limitations are discussed.

An 18-channel multifrequency laser

Abstract: A multifrequency laser (MFL) is demonstrated that runs simultaneously CW on 18 channels spaced by 103 GHz. The laser emits -14.6-dBm power per wavelength channel into single-mode fiber. Each wavelength channel can be modulated at 1.24 Gb/s. The MFL exhibits a stable and reproducible optical channel spacing owing to the reproducibility of the waveguide grating router that serves as the intracavity filter element.

Impact of double Rayleigh backscatter noise on digital and analog fiber systems

Abstract: We have derived, and experimentally verified, an exact analytical expression for the noise power spectral density due to double Rayleigh backscattering in standard single mode fiber-optic transmission systems in terms of the autocorrelation function of the transmitted optical electric field. The analysis is used to examine the impact of double Rayleigh backscattering on chirped amplitude modulated-subcarrier multiplexed (AM-SCM), and optically amplified digital intensity modulated-direct detection (IM-DD) laser systems, with and without optical isolators in the link.

Modeling of four-wave mixing and gain peaking in amplified WDM optical communication systems and networks

Abstract: A theoretical model is presented for analyzing the propagation of densely spaced WDM optical signals through a cascade of erbium-doped fiber amplifiers and single-mode optical fibers with nonuniform chromatic dispersion. By combining a numerical solution for the EDFA and an analytical expression for FWM components generated through the cascade, the model allows a realistic system analysis which includes gain peaking effect, amplified spontaneous emission accumulation and the effect of dispersion management on the four-wave mixing efficiency. The FWM power distribution at the end of the multi-amplifier transmission link is computed taking into account the phase relation between FWM light amplitudes generated within different sections of the link. The transmission of many WDM channels, evenly spaced around 1547.5 nm, has been analyzed for various dispersion management techniques and propagation distances. Numerical results point out the importance of such a model for a realistic design of WDM optical communication systems and networks. A proper choice of chromatic dispersion, amplifier characteristics, span length, input signal powers and wavelengths, combined with the use of gain equalizing filters, allows to maximize the transmission distance ensuring acceptable signal-to-noise ratio (SNR) and limited SNR variation among channels.

Optical source with mode reshaping

Abstract: A laser cavity (90) configuration employs a mode reshaping structure (430) in a solid state waveguide (132). The mode reshaping may be by means of an adiabatic taper (430). Polarization switching may be employed in connection with the mode reshaping to obtain a high output power in a single transverse spatial mode from an integrated external cavity multiple mode diode laser device (440). The structure provides stabilization and rapid tuning (120) of the frequency of the diode laser, as well as use substantially all of the avalaible power of the diode laser operating mode. Various configurations and combinations are described. An advantage is that an integrated structure can provide output of the laser in a waveguide (132) with single mode for subsequent applications. The external cavity diode laser (90) according to the invention is consistent with a planar manufacturing process such that high-volume, low-cost products can be achieved.

Pure silica single-mode fibre with hexagonal photonic crystal cladding

Abstract: Pure silica fibres supporting guided modes were first investigated in the 1970's [ 11, the aim being to achieve low transmission losses. The huge success of chemical vapour deposition in producing extremely low loss fibre has largely superseded this early technology. We have recently revisited it in the context of photonic crystals [2], and report here the realisation of a new kind of pure silica microstructured optical fibre which supports a robust single mode. Photonic crystals are periodically microstructured materials with a pitch on the scale of the optical wavelength. They have recently been the subject of much interest because of their unusual optical properties, including their ability to support a full photonic band gap [3]. Severa1 research teams have reported fabricating two-dimensional photonic crystal material out of glasses using selective etching processes [4,5]. However, such a fabrication process results in samples of at most a few millimetres in the third dimension. The photonic crystal fibre described here is formed by creating a hexagonal silica/air preform (including a deliberate defect to guide light) on a macroscopic scale and then reducing its size by several orders of magnitude by pulling it into an optical fibre (see Figure 1).

Breathing solitons in optical fiber links

Abstract: We introduce the concept of “breathing” solitons to describe the dynamics of optical pulses in transmission lines with passive compensation of fiber chromatic dispersion. The “breathing” pulse can be used as the information carrier. The theory presented is complimentary to the concept of the guiding-center soliton. It is shown that an average bright soliton can propagate in a system with large variations of the dispersion, including segments with high normal dispersion.

Efficient electromechanical optical switches

Abstract: The present invention provides improved optic switches in which the optic fibers and optical pathways need not move. Advantageously, the switches of the present invention generally rely on a combination of a moveable reflective element and at least one fixed collimating lens. The collimating lens typically expands the light signal from a single mode fiber to a substantially larger optic path. When the reflective element is disposed out of this large optic path, the light signals continue on to a first output fiber, often through another collimating lens. However, when the reflective element is disposed within the expanded optic path from the collimating lens, the signal is reflected back through that same collimating lens into an alternative output fiber which is parallel and in close proximity to the input fiber. Conveniently, the reflective element can move across the optic path without changing the position of the input or output fibers relative to each other. Surprisingly, a thin planar reflector which is aligned to reflect one of a pair of signals crossing between two collimating lenses can also reflect the other crossing signal, thereby providing highly efficient multiple signal switching. Hence, by carefully locating a number of optic fibers parallel to each other about the center line of the collimating lens, a wide variety of highly efficient, multiple input and multiple output switching structures can be constructed.

Single-mode nonlinear-optical polymer fibers

Abstract: We report on the successful demonstration of a single-mode polymer-optical fiber with an 8-µm-diameter nonlinear-optical core composed of a dye-chromophore-doped polymer. Both solid-solution cores and copolymer cores were successfully fabricated. Using an imaging system, we show that the far-field transverse light pattern is that of a single-mode guide. We find that the loss at 1064 nm for the single-mode fiber is approximately 0.2 dB/cm and that it preserves polarization to better than 99.8%/cm.

Transverse strain measurements using fiber optic grating based sensors

Abstract: A system and method to sense the application of transverse stress to an optical fiber which includes a light source that producing a relatively wide spectrum light beam. The light beam is reflected or transmitted off of an optical grating in the core of an optical fiber that is transversely stressed either directly or by the exposure to pressure when the fiber is bifringent so that the optical fiber responds to the pressure to transversely stress its core. When transversely stressed, the optical grating produces a reflection or transmission from the light beam that has two peaks or minimums in its frequency spectrum whose spacing and/or spread are indicative of the forces applied to the fiber. One or more detectors sense the reflection or transmissions from the optical grating to produce an output representative of the applied force. Multiple optical gratings and detectors may be employed to simultaneously measure temperature or the forces at different locations along the fiber.

Using an asymmetric element to create a 1XN optical switch

Abstract: An architecture for a compact, 1×N optical switch. The switch package receives light from an input optical fiber (12b), which is directed over a well or gap at the bottom of which lies a micromechanical structure (10). If the structure is in an unaddressed state, the light travels into an in line output optical fiber (12a). If the structure (10) is in an addressed state, it intercepts the light and reflects it out of the plane of the input optical fiber to an offset mirror (24). The offset mirror (24) then reflects the light to one output fiber (16a, 16b). The offset mirror may have steps such that more than one optical fiber could become the output fiber, depending upon the structure's position.

High intensity single-mode VCSELs

Abstract: A VCSEL 101 comprising an optical cavity having an optical loss and a loss-determining element 117 coupled to the optical cavity. The loss-determining element 117 progressively increases the optical loss of the optical cavity with increasing lateral distance from the optical axis 105. The optical cavity includes a first mirror region 111, a second mirror region 107, a plane light-generating region 125 sandwiched between the first mirror region 111 and the second mirror region 107, perpendicular to the optical axis 105, and an element 113 that defines the lateral extent of the optical cavity in the plane of the light-generating region 125. The first mirror region 111 and the second mirror region 107 are both conductive and have opposite conductivity modes.

Transmission of 60-GHz signals over 100 km of optical fiber using a dual-mode semiconductor laser source

Abstract: High-purity 60-GHz signals, generated using a dual-mode DFB semiconductor laser, have been transmitted over 100 km of standard optical fiber with no observable degradation in purity. This device is shown to be a simple and compact optical source of high-purity 60-GHz signals for long-reach fiber-fed millimeter-wave radio systems.

Single cavity solid state laser with intracavity optical frequency mixing

Abstract: A composite cavity continuous wave (cw) microlaser that lases at two fundamental wavelengths, denoted by λ 1 and λ 2 , which are frequency-mixed in a suitable nonlinear crystal oriented within an optically resonant cavity for phase-matched frequency mixing to generate radiation at a third wavelength. The optically resonant cavity is defined by a first reflective surface and a second reflective surface, both of which are substantially reflective at a first and a second wavelength. A highly absorbing solid-state gain material, preferably Nd:YVO 4 , which has a first gain transition at the first wavelength and a second gain transition at the second wavelength different from the first wavelength is disposed within the optically resonant cavity. The highly absorbing solid-state gain material is closely coupled to the first reflective surface to promote single mode operation of both fundamental lasing frequencies. An optical pump source is optically coupled through the first reflective face to end-pump the solid-state gain material with continuous pump radiation at a pump wavelength that is highly absorptive by the gain material. In some embodiments, the solid-state gain material has an input face that includes the first reflective surface and the nonlinear crystal has an output face that includes the second reflective surface.

Current-apertured vertical-cavity laser

Abstract: A vertical-cavity surface-emitting laser (VCSEL) has an active region (20), first and second mirror stacks (14, 26) forming a resonant cavity with a radial variation in index forming a transverse optical mode (32), and a thin insulating slot (27) within the cavity to constrict the current to a diameter less than the beam waist of the optical mode thereby improving device efficiency and preferentially supporting single mode operation. In one embodiment, an insulating slot is formed by etching or selectively oxidizing a thin aluminium-containing semiconductor layer in towards the center of a cylindrical mesa. The slot thickness is sufficiently thin that the large index discontinuity has little effect on the transverse optical-mode pattern. The slot may be placed near an axial standing-wave null to minimize the perturbation of the index discontinuity and allow the use of thicker slots. In a preferred embodiment, the current constriction, formed by the insulating slot, is located on the p-type side of the active region and has a diameter significantly less than the beam waist of the optical mode, thus minimizing outward diffusion of carriers and ensuring single transverse-mode operation of the laser by suppressing spatial hole burning.

Phase shifting diffraction interferometry for measuring extreme ultraviolet optics

Abstract: Extreme ultraviolet projection lithography operating at a wavelength of 13nm requires surface figure accuracy on each mirror to be better than 0.25nm rms. A new type of interferometry, based on the fundamental process of diffraction, is described that can intrinsically achieve the required accuracy. Applying this principle, two independent spherical wavefronts are generated - one serves as the measurement wavefront and is incident on the optic or optical system under test and the other serves as the reference wavefront. Since they are generated independently their relative amplitude and phase can be controlled, providing contrast adjustment and phase shifting capability. Using diffraction from a single mode optical fiber, different interferometers can be configured to measure individual mirrors or entire imaging systems. Measurement of an EUV projection system is described. 5 refs., 4 figs.

Widely tunable spectrum translation and wavelength exchange by four-wave mixing in optical fibers

Abstract: Summary form only given. Wavelength conversion has been the subject of much research in recent years. Various methods have been studied with use of optical nonlinearities in either semiconductors or fibers. We introduce a novel technique based on four-wave mixing (FWM) in fibers, which has the potential for achieving unit conversion efficiency. It involves only spectrum translation, not inversion or phase conjugation. It is widely tunable and can, in principle, shift wavelengths by tens and even hundreds of nanometers. It can also perform a novel optical function, namely, complete exchange of optical power between two wavelengths.

Travelling wave model of a multimode Fabry-Perot laser in free running and external cavity configurations

Abstract: We report the results of a numerical study of multimode behaviour of a Fabry-Perot laser. The model is based on travelling-wave equations for the slowly varying amplitudes of the counterpropagating waves in the cavity, coupled to equations for spatially dependent population inversion and polarization of a two-level active medium. Variations in the material variables on the scale of a wavelength are taken into account by means of an expansion in a Fourier series. Results are given for typical semiconductor laser parameters. Spatially distributed spontaneous emission noise and carrier diffusion are taken into account. The competing roles of spatial hole burning (SHE), spontaneous emission noise, and carrier diffusion in determining multimode behaviour are elucidated; with no carrier diffusion, spontaneous emission noise excites a large number of modes close to threshold, while SHE leads to a fixed number of significant lasing modes well above threshold. Carrier diffusion washes out the gratings in the material variables, and the resulting strengthening of the inter-mode coupling (cross-saturation) restores dominant single-mode emission well above threshold. We have also studied the effects of optical feedback and opportunities for mode selection with short external cavities; for an external cavity much shorter than the laser cavity length and a small field amplitude reflectivity coefficient, a single mode can be selected. For a large reflectivity coefficient, two groups of intracavity modes separated by the external cavity mode interspacing are selected. For an external cavity with a round trip time half that of the laser cavity, the laser can be forced with modest feedback to operate on two modes that are both quasiresonant with the external cavity. Mode selection is not found, even for weak feedback, when the external mode spacing is about 90% of the laser mode spacing.

Limitations in 10 Gb/s WDM optical-fiber transmission when using a variety of fiber types to manage dispersion and nonlinearities

Abstract: We analyze the system limitations of WDM transmission when using various types of optical fiber to manage dispersion and nonlinearities. In our model, from two to eight 10 Gb/s WDM channels are transmitted through a cascade of EDFA's experiencing dispersion, stimulated Raman scattering, and self- and cross-phase modulation. The fiber types modeled include: conventional single-mode fiber, dispersion shifted fiber, and dispersion-compensating fiber. These fibers have different dispersion spectral profiles and are combined to manage dispersion to produce a total zero dispersion for a certain fiber span while eliminating four-wave mixing. We find that a system using dispersion-shifted fiber and conventional single-mode fiber exhibits the best performance, with the combination of dispersion and cross-phase modulation as the dominant effects. Furthermore, conventional single-mode fiber combined with dispersion-compensating fiber system exhibits the worst performance, with the combination of dispersion and self-phase modulation as the dominant effects.