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

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

Nonlinear photonic chips can generate and process signals all-optically with far superior performance to that possible electronically — particularly with respect to speed. Although silicon-on-insulator has been the leading platform for nonlinear optics, its high two-photon absorption at telecommunication wavelengths poses a fundamental limitation. We review recent progress in non-silicon CMOS-compatible platforms for nonlinear optics, with a focus on Si3N4 and Hydex®. These material systems have opened up many new capabilities such as on-chip optical frequency comb generation and ultrafast optical pulse generation and measurement. We highlight their potential future impact as well as the challenges to achieving practical solutions for many key applications. This article reviews recent progress in the use of silicon nitride and Hydex as non-silicon-based CMOS-compatible platforms for nonlinear optics. New capabilities such as on-chip optical frequency comb generation, ultrafast optical pulse generation and measurement using these materials, and their potential future impact and challenges are covered.

New CMOS-compatible platforms based on silicon nitride and hydex for nonlinear optics

Today cross-cutting approaches, where molecular engineering and clever processing are synergistically coupled, allow the chemist to tailor complex hybrid systems of various shapes with perfect mastery at different size scales, composition, functionality, and morphology. Hybrid materials with organic–inorganic or bio–inorganic character represent not only a new field of basic research but also, via their remarkable new properties and multifunctional nature, hybrids offer prospects for many new applications in extremely diverse fields. The description and discussion of the major applications of hybrid inorganic–organic (or biologic) materials are the major topic of this critical review. Indeed, today the very large set of accessible hybrid materials span a wide spectrum of properties which yield the emergence of innovative industrial applications in various domains such as optics, micro-electronics, transportation, health, energy, housing, and the environment among others (526 references).

Applications of advanced hybrid organic–inorganic nanomaterials: from laboratory to market

The nonlinearities in silicon are diverse. This Review covers the wealth of nonlinear effects in silicon and highlights the important applications and technological solutions in nonlinear silicon photonics. The increasing capability for manufacturing a wide variety of optoelectronic devices from polymer and polymer–silicon hybrids, including transmission fibre, modulators, detectors and light sources, suggests that organic photonics has a promising future in communications and other applications.

Nonlinear silicon photonics

An optical sampling oscilloscope is used to characterize short pulses which have been shaped using superstructured FBGs. The performance of square-pulse-generating and pulse-multiplying gratings are examined directly in real time, with a resolution of 2ps, excellent pulse quality is confirmed.

A direct assessment of the performance of pulse shaping superstructured fiber gratings using an optical sampling oscilloscope

Wavelength conversion is a key WDM function which is used to avoid wavelength blocking, thus allowing greater flexibility in the network design. Wavelength conversion of an intensity modulated signal can be simply achieved by employing cross-phase modulation (XPM) on a CW beam placed at the desired wavelength, normally followed by a band-pass filter and a narrowband notch filter to reject the pump and the carrier frequency respectively. The notch filter in particular has to be precisely tuned to the CW wavelength and be extremely narrowband in order to avoid any distortions on the spectral envelope of the converted signal (see e.g. [1]). This extra complexity could be avoided using conventional band-pass filters only to select a single sub-carrier band, but this usually implies non optimal CW extinction and thus some extra power penalties as compared to the original signal. In this paper we investigate the use of pulse shaping of the signal to be converted into a saw-tooth waveform (i.e. a pulse with leading/trailing edges of a constant intensity gradient), to improve the performance of the converted signal. Due to their linear gradient, when saw-tooth pulses are used as the XPM pump, they induce a constant frequency shift to the signal [2, 3]. These new frequency components are highly spectrally confined and discretely separated from the CW carrier, thus allowing most of the wavelength converted signal energy to pass through the subsequent offset filters, thereby improving the overall optical signal to noise ratio (OSNR) and the overall system performance. Similarly, self-phase modulation (SPM) of saw-tooth pulses is more confined, giving more flexibility on the choice of the pump and signal wavelength allocations.

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Efficient all-optical wavelength converter using saw-tooth pulses

Progress in optical communications has been one of the key factors for the enormous growth of the ICT sector, and, in particular, of the Internet phenomenon. Such a progress has been driven by experimental successes that have been obtained in the last three decades in several laboratories all over the world, and we have witnessed a fantastic challenge among such labs to reach record targets such as maximum bit rate, maximum propagation distance, higher performance, maximum efficiency, and, recently, minimum energy consumption. The search for the maximum bit rate × distance was based on the principle of infinite bandwidth of the optical fiber that let us to imagine transmission of enormous capacities over transoceanic distances, especially after the invention of the optical amplifier.

Experiments on Long-Haul High-Capacity Transmission Systems

We experimentally demonstrate an OTDM demultiplexer based on a fiber Kerr gate which uses a linearly- chirped rectangular control pulse generated by shaping a spectrally broadened pulse with a fiber Bragg grating. Error- free, 40Gbit/s to 10Gbit/s demultiplexing is successfully achieved.

All-optical TDM data demultiplexing based on a highly nonlinear fiber kerr gate using a linearly chirped rectangular control pulse

Phase noise introduced during transmission both from optical amplifiers and nonlinear interactions between channels represents a significant limiting factor to data transmission when advanced modulation formats such as (differential) phase-shift keying, (D)PSK, or quadrature phase shift keying, QPSK, signals are used to increase the network capacity [1]. Consequently, the development of all-optical techniques capable of eliminating phase (and ideally amplitude as well) noise from multi-level phase signals is of great interest. Phase regeneration of (D)PSK signals can be achieved directly by exploiting the phase-squeezing capability of phase sensitive amplifiers (PSAs) [2], which can be operated in the saturation regime to perform simultaneous amplitude regeneration. The main challenge in realising practical PSA-based regeneration is to stabilize and maintain a phase relationship between the PSA pump(s), the signal and any idlers present at the PSA input. This is complicated in practice by the fact that phase-encoded signals have a suppressed carrier. Moreover, even if the carrier can be extracted, it would generally incorporate any noise generated during data transmission. To overcome these issues a regenerative scheme has recently been demonstrated [3“5] which simultaneously allows carrier recovery and carrier noise suppression. The resulting cleaned carrier is then used to phase-lock the locally generated pumps with the incoming data prior to signal regeneration in a degenerate two-pump PSA [4–5]. In this scheme two important technologies were used: injection locking of semiconductor lasers to remove high frequency amplitude and phase noise during the carrier recovery process [6] and specialized highly nonlinear fibres (HNLFs) with an alumino-silicate core and a linear strain gradient applied along the length to increase the stimulated Brillouin scattering (SBS) threshold [7] in order to allow higher PSA gains. Figures 1 (a) and (b) show example constellation diagrams at the input/output of the regenerator for noisy input DPSK signals at 56Gbaud illustrating the regenerative performance achieved.

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Francesca Parmigiani

Papers

A direct assessment of the performance of pulse shaping superstructured fiber gratings using an optical sampling oscilloscope

Efficient all-optical wavelength converter using saw-tooth pulses

Experiments on Long-Haul High-Capacity Transmission Systems

All-optical TDM data demultiplexing based on a highly nonlinear fiber kerr gate using a linearly chirped rectangular control pulse

Potential and practical implementations of phase sensitive amplifiers for all-optical signal regeneration