[신간의 별자리x] 우리/미술, 그리고 ‘슬픔의 박물관’

This Review summarizes the simultaneous transmission of several independent spatial channels of light along optical fibres to expand the data-carrying capacity of optical communications. Recent results achieved in both multicore and multimode optical fibres are documented.

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Space-division multiplexing in optical fibres

Several kinds of nonlinear optical effects have been observed in recent years using silicon waveguides, and their device applications are attracting considerable attention. In this review, we provide a unified theoretical platform that not only can be used for understanding the underlying physics but should also provide guidance toward new and useful applications. We begin with a description of the third-order nonlinearity of silicon and consider the tensorial nature of both the electronic and Raman contributions. The generation of free carriers through two-photon absorption and their impact on various nonlinear phenomena is included fully within the theory presented here. We derive a general propagation equation in the frequency domain and show how it leads to a generalized nonlinear Schrodinger equation when it is converted to the time domain. We use this equation to study propagation of ultrashort optical pulses in the presence of self-phase modulation and show the possibility of soliton formation and supercontinuum generation. The nonlinear phenomena of cross-phase modulation and stimulated Raman scattering are discussed next with emphasis on the impact of free carriers on Raman amplification and lasing. We also consider the four-wave mixing process for both continuous-wave and pulsed pumping and discuss the conditions under which parametric amplification and wavelength conversion can be realized with net gain in the telecommunication band.

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Nonlinear optical phenomena in silicon waveguides: Modeling and applications

Significant effort in optical-fibre research has been put in recent years into realizing mode-division multiplexing (MDM) in conjunction with wavelength-division multiplexing (WDM) to enable further scaling of the communication bandwidth per fibre. In contrast, almost all integrated photonics operate exclusively in the single-mode regime. MDM is rarely considered for integrated photonics because of the difficulty in coupling selectively to high-order modes, which usually results in high inter-modal crosstalk. Here we show the first microring-based demonstration of on-chip WDM-compatible mode-division multiplexing with low modal crosstalk and loss. Our approach can potentially increase the aggregate data rate by many times for on-chip ultrahigh bandwidth communications.

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WDM-compatible mode-division multiplexing on a silicon chip

Space-division multiplexing (SDM) uses multiplicity of space channels to increase capacity for optical communication. It is applicable for optical communication in both free space and guided waves. This paper focuses on SDM for fiber-optic communication using few-mode fibers or multimode fibers, in particular on the critical challenge of mode crosstalk. Multiple-input–multiple-output (MIMO) equalization methods developed for wireless communication can be applied as an electronic method to equalize mode crosstalk. Optical approaches, including differential modal group delay management, strong mode coupling, and multicore fibers, are necessary to bring the computational complexity for MIMO mode crosstalk equalization to practical levels. Progress in passive devices, such as (de)multiplexers, and active devices, such as amplifiers and switches, which are considered straightforward challenges in comparison with mode crosstalk, are reviewed. Finally, we present the prospects for SDM in optical transmission and networking.

Space-division multiplexing: the next frontier in optical communication

We demonstrate mode-division multiplexed WDM transmission over 50-km of few-mode fiber using the fiber’s LP01 and two degenerate LP11 modes. A few-mode EDFA is used to boost the power of the output signal before a few-mode coherent receiver. A 6×6 time-domain MIMO equalizer is used to recover the transmitted data. We also experimentally characterize the 50-km few-mode fiber and the few-mode EDFA.

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Mode-division multiplexed transmission with inline few-mode fiber amplifier.

We propose a criterion to predict the relative value of the stimulated Brillouin scattering (SBS) threshold in single-mode optical fibers with different refractive index profiles. We confirm our results by several representative measurements. We show that with the proper profile design one can achieve more than 3 dB increase in the SBS threshold compared to the standard single-mode optical fiber.

Design concept for optical fibers with enhanced SBS threshold

An optical fiber according to an embodiment of the present invention comprises: a glass core extending from a centerline to a radius R1 wherein R1 is greater than about 5 µm; a glass cladding surrounding and in contact with the core, the cladding comprising: (i) a first annular region extending from the radius R1 to a radius R2 the first annular region comprising a radial width, W2 = R2 - R1, (ii) a second annular region extending from the radius R2 to a radius R3, and comprising a radial width, W3 = R3 - R2, and (iii) a third annular region surrounding the second annular region and extending from the radius R3 to an outermost glass radius R4; wherein the core comprises a maximum relative refractive index, Δ1max, relative to the third annular region, and wherein Δ1MAX is greater than about 0.1 % and less than about 0.3 %; the first annular region has a refractive index delta Δ2(r) is less than about 0.025%; wherein the second annular region comprises a minimum relative refractive index, Δ3MIN, relative to the third annular region; wherein Δ1MAX > Δ2MAX > Δ3MIN, and Δ2MIN > Δ3MIN<0; and wherein the core and the cladding provide a fiber with cable cutoff less than 1500 nm, and an effective area at 1550 nm greater than 95 µm2 and bend loss of =≤0.5 dB/turn on a 20 mm diameter mandrel.

Large effective area fiber

Bend resistant optical fibers which are multi-moded at 1300 nm include a core, an inner cladding, a low index ring and an outer cladding. The core has a graded index of refraction with a core alpha profile where 1.9≦αC≦2.1, a maximum relative refractive index percent Δ1Max%, and a numerical aperture NA of greater than 0.23. The inner cladding surrounds the core and has a maximum relative refractive index percent Δ2Max%, a minimum relative refractive index percent Δ2Min%, and a radial thickness ≧0.5 microns, wherein Δ1Max%>Δ2Max%. The low index ring surrounds the inner cladding and has a relative refractive index percent Δ3%, a radial thickness of at least 0.5 microns, a profile volume with an absolute magnitude of greater than 50%-μm2, wherein Δ2Min%≧Δ3%. The outer cladding surrounds the low index ring and has a relative refractive index percent Δ4%, such that Δ1Max%>Δ4%≧Δ2Max%.

Large numerical aperture bend resistant multimode optical fiber

Optical waveguide fiber that has large effective area and low loss characteristics, such as low attenuation and low bend loss. The optical waveguide fiber includes a dual trench design wherein an annular region closer to the core is preferably doped with at least one downdopant such as fluorine, which annular region is surrounded by another annular region that preferably includes closed, randomly dispersed voids.

Scott R. Bickham

Papers

Mode-division multiplexed transmission with inline few-mode fiber amplifier.

Design concept for optical fibers with enhanced SBS threshold

Large effective area fiber

Large numerical aperture bend resistant multimode optical fiber

Large Effective Area Low Attenuation Optical Fiber