Researchers demonstrate the ability to multiplex and transfer data between twisted beams of light with different amounts of orbital angular momentum — a development that provides new opportunities for increasing the data capacity of free-space optical communications links.

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Terabit free-space data transmission employing orbital angular momentum multiplexing

We describe a method to estimate the capacity limit of fiber-optic communication systems (or ?fiber channels?) based on information theory. This paper is divided into two parts. Part 1 reviews fundamental concepts of digital communications and information theory. We treat digitization and modulation followed by information theory for channels both without and with memory. We provide explicit relationships between the commonly used signal-to-noise ratio and the optical signal-to-noise ratio. We further evaluate the performance of modulation constellations such as quadrature-amplitude modulation, combinations of amplitude-shift keying and phase-shift keying, exotic constellations, and concentric rings for an additive white Gaussian noise channel using coherent detection. Part 2 is devoted specifically to the "fiber channel.'' We review the physical phenomena present in transmission over optical fiber networks, including sources of noise, the need for optical filtering in optically-routed networks, and, most critically, the presence of fiber Kerr nonlinearity. We describe various transmission scenarios and impairment mitigation techniques, and define a fiber channel deemed to be the most relevant for communication over optically-routed networks. We proceed to evaluate a capacity limit estimate for this fiber channel using ring constellations. Several scenarios are considered, including uniform and optimized ring constellations, different fiber dispersion maps, and varying transmission distances. We further present evidences that point to the physical origin of the fiber capacity limitations and provide a comparison of recent record experiments with our capacity limit estimation.

Capacity Limits of Optical Fiber Networks

Orbital angular momentum (OAM), which describes the “phase twist” (helical phase pattern) of light beams, has recently gained interest due to its potential applications in many diverse areas. Particularly promising is the use of OAM for optical communications since: (i) coaxially propagating OAM beams with different azimuthal OAM states are mutually orthogonal, (ii) inter-beam crosstalk can be minimized, and (iii) the beams can be efficiently multiplexed and demultiplexed. As a result, multiple OAM states could be used as different carriers for multiplexing and transmitting multiple data streams, thereby potentially increasing the system capacity. In this paper, we review recent progress in OAM beam generation/detection, multiplexing/demultiplexing, and its potential applications in different scenarios including free-space optical communications, fiber-optic communications, and RF communications. Technical challenges and perspectives of OAM beams are also discussed.

Optical communications using orbital angular momentum beams

The very broad bandwidth of low-loss optical transmission in a single-mode fiber and the recent improvements in single-frequency tunable lasers have stimulated significant advances in dense wavelength division multiplexed optical networks This technology, including wavelength-sensitive optical switching and routing elements and passive optical elements, has made it possible to consider the use of wavelength as another dimension, in addition to time and space, in network and switch design The independence of optical signals at different wavelengths makes this a natural choice for multiple-access networks, for applications which benefit from shared transmission media, and for networks in which very large throughputs are required Recent progress in multiwavelength networks are reviewed, some of the limitations which affect the performance of such networks are discussed, and examples of several network and switch proposals based on these ideas are presented Discussed also are critical technologies that are essential to progress in this field >

Dense wavelength division multiplexing networks: principles and applications

Since the discovery of photosensitivity in optical fibers there has been great interest in the fabrication of Bragg gratings within the core of a fiber. The ability to inscribe intracore Bragg gratings in these photosensitive fibers has revolutionized the field of telecommunications and optical fiber based sensor technology. Over the last few years, the number of researchers investigating fundamental, as well as application aspects of these gratings has increased dramatically. This article reviews the technology of Bragg gratings in optical fibers. It introduces the phenomenon of photosensitivity in optical fibers, examines the properties of Bragg gratings, and presents some of the important developments in devices and applications. The most common fabrication techniques (interferometric, phase mask, and point by point) are examined in detail with reference to the advantages and the disadvantages in utilizing them for inscribing Bragg gratings. Reflectivity, bandwidth, temperature, and strain sensitivity of the Bragg reflectors are examined and novel and special Bragg grating structures such as chirped gratings, blazed gratings, phase-shifted gratings, and superimposed multiple gratings are discussed. A formalism for calculating the spectral response of Bragg grating structures is described. Finally, devices and applications for telecommunication and fiber-optic sensors are described, and the impact of this technology on the future of the above areas is discussed.

Fiber Bragg gratings

We demonstrate a one-dimensional grating coupler in silicon nitride with a 67-nm 1-dB bandwidth, the largest reported for a grating coupler butt-coupled to standard single-mode fiber. The peak coupling efficiency is -4.2dB. It requires no partial etch and requires only 0.6-μm fabrication resolution.

Wide Bandwidth Silicon Nitride Grating Coupler

We have integrated a broadly tunable grating‐assisted vertical coupler as an intracavity filter to demonstrate a novel monolithic multiple‐quantum‐well InGaAsP/InP laser. The narrow, bandpass intracavity filter results in an extended cavity laser with a measured electrical tuning range of 57 nm with single‐frequency operation over most of that range.

Broadly tunable InGaAsP/InP laser based on a vertical coupler filter with 57‐nm tuning range

We report a novel wavelength selective coupler based upon grating‐assisted forward coupling between two, nonidentical, vertically stacked InGaAsP/InP waveguides. Using a 14.4 μm period to provide phase matching between the asynchronous waveguides, we achieve a filter bandwidth (full width at half maximum) of 65 A and coupling efficiency of ∼65% with a device length of only 1.0 mm.

Grating‐assisted InGaAsP/InP vertical codirectional coupler filter

We demonstrated a 40-channel dense-wavelength-division-multiplexed (DWDM) monolithically integrated receiver on silicon, which consists of a silicon nitride (Si3N4)arrayed waveguide grating (AWG) demultiplexer with integrated germanium-on-silicon waveguide photodetectors. Net responsivities from a cleaved standard single-mode fiber to detectors of 0.15 ~ 0.22 A/W are measured, with flattened passbands with 1-dB bandwidth over 60% of the 200-GHz channel spacing. Single-channel operation up to 40 Gb/s is demonstrated with a SiGe transimpedance amplifier.

Monolithically Integrated 40-Wavelength Demultiplexer and Photodetector Array on Silicon

We report the first demonstration of efficient narrowband optical wavelength filters using InGaAsP/InP passive waveguide grating resonators. Filter bandwidths as narrow as 1 A, centered about λ=1.55 μm with excess resonator loss as low as 1 dB, have been achieved.

L. L. Buhl

Papers

Wide Bandwidth Silicon Nitride Grating Coupler

Broadly tunable InGaAsP/InP laser based on a vertical coupler filter with 57‐nm tuning range

Grating‐assisted InGaAsP/InP vertical codirectional coupler filter

Monolithically Integrated 40-Wavelength Demultiplexer and Photodetector Array on Silicon

Narrowband grating resonator filters in InGaAsP/InP waveguides