Multilevel Modulation Formats Research Articles

Modulation format conversion between BPSK and 8QAM signals using coherent interference and four-wave mixing

Optical fiber networks need to transmit various types of client data with different requirements of bandwidth, reach, latency, and capacity by interconnecting backbone, metro, and local access networks. Since the optimal modulation format usually differs, it should be efficiently converted at the intermediate node connecting different networks. Among the multilevel modulation format, 8-ary quadrature amplitude modulation (8QAM) is a candidate for terrestrial networks thanks to the good balance between spectral efficiency and transmission distance. In this paper, we propose modulation format conversion systems from binary phase shift keying (BPSK) to 8QAM using coherent interference in delay interferometer and its reverse conversion system using four-wave mixing in a single highly nonlinear fiber. Conversion performances are numerically evaluated based on bit error rate, constellation diagrams, 8QAM and BPSK propagation lengths, and parameters in electrical signal regeneration. As a result, error-free format conversion is achieved for both conversion systems.

Chirped Managed Laser for Multilevel Modulation Formats: A Semi-Analytical Approach for Efficient Filter Design

Chirped Managed Laser for Multilevel Modulation Formats: A Semi-Analytical Approach for Efficient Filter Design

260-GBaud Single-Wavelength Coherent Transmission Over 100-km SSMF Based on Novel Arbitrary Waveform Generator and Thin-Film Niobate I/Q Modulator

In this paper, we demonstrate up to 260-GBaud single-wavelength coherent transmission by employing an optical transmitter based on two wide-bandwidth devices: a novel 260-GS/s arbitrary waveform generator with a 10-dB bandwidth of 90-GHz and a thin-film Lithium Niobate I/Q modulator with a 3-dB bandwidth of 110-GHz. We first assess the performance of our high symbol rate transmitter for generating spectrally efficient Nyquist multilevel modulation format signals at symbol rates up to 210-GBaud with >1.4-Tbps achievable information rates and by using linear digital signal processing only. We achieve up to 1.84-Tbps at 185-GBaud using PCS-64QAM, highlighting the linear behavior of our transmitter for high symbol rate Nyquist signaling. We then switch to dual-polarization QPSK coherent transmission and further increase the symbol rate up to 260-GBaud. Without using nonlinear digital-signal-processing nor advanced inter-symbol-interference mitigation techniques, we successfully transmit 260-GBaud QPSK at 800-Gbps net rate over 100-km of standard single mode fiber.

Comparative analysis of 128 × 40 Gbps DQPSK based DWDM long-haul system using various dispersion compensation techniques

Abstract In this research article, we have made the comparative performance analysis towards the ultra-transmission capacity of 5.12 Tbps for DWDM long-haul system using spectrally efficient multilevel DQPSK modulation format under the influence of Kerr’s nonlinearities with DCF based distributed, pre-, post and two types of symmetrical compensation techniques. The performance evaluation criteria of the proposed system for 50 GHz frequency spacing between the adjacent channels at 0 dBm launched power and various transmission distances with the increment of 2400 km are kept as BER, Q-factor and eye-diagrams. We have used Gaussian filters (GFs) at both ends before multiplexing and after de-multiplexing for ensuring channel severance to elude the cross-talk and inter-symbol interference. It is observed that the distributed compensation technique outperformed rest of the techniques and supported maximum coverage distance towards 19,200 km for Q-factor of 6.96 and corresponding BER of 1.72E-12 in comparison to sym-1 for 14,400 km at Q-factor of 6.95 with BER of 1.77E-12 and sym-2 for 9600 km at Q-factor of 7.13 with BER of 4.83E-13 whereas both pre- and post-compensation techniques supported 7200 km at Q-factor of 7.21 with BER of 2.71E-13 and Q-factor of 7.18 for BER of 3.53E-13, respectively.

Terahertz fiber with multi-concentric ring cores for OAM modes propagation

A novel fiber incorporating central hollow, porous isolated layers, and concentric ring cores is proposed for the simultaneous propagation of multi-terahertz (THz) orbital angular momentum (OAM) modes with low-level inter-core and inter-mode crosstalk. The designed fiber can efficiently support 132 OAM modes in 0.6 ∼ 1.5 THz, 178 OAM modes in 0.7 ∼ 1.5 THz, etc, the high-order radial modes are suppressed within the whole frequency range meanwhile, and the number of OAM modes can be further boosted by further increasing the number of ring cores. In addition, the fiber has low confinement loss, flat dispersion, and high purity over a wide operating range. Hence it can be applied in mode-division multiplexing (MDM) based on OAM combined with core-division multiplexing (CDM) in THz range, and is also compatible with wavelength-division multiplexing (WDM) and multi-level modulation formats. The realized fiber is expected to dramatically extend the transmission capacity and spectral efficiency.

Mitigation of PAM4 rolling shuttered pattern grayscale ambiguity in demodulation utilizing long short term memory neural network (LSTM-NN) in optical wireless communication systems

One realization of optical wireless communication (OWC) is optical camera communication (OCC), which utilizes mobile-phone and surveillance cameras for communication. One main limitation of OCC system is the low frame rate of typical cameras. To enhance the OCC data rate with limited frame rate cameras, rolling shuttered camera mode can be utilized. However, decoding the rolling shuttered pattern is very challenging. In addition, utilizing multi-level modulation formats, such as four-level pulse-amplitude-modulation (PAM4) can increase the OCC bit-per-symbol; however, it would be a great challenge to identify the multiple intensity levels owing to the uneven aggregated light exposure of each pixel-row. In this work, we put forward and demonstrate utilizing the Long-Short-Term Memory neural-network (LSTM-NN) to alleviate the decoding of high inter-symbol-interference (ISI) of PAM4 rolling shuttered pattern. The experiment is performed in a practical room with 3-m free-space transmission distance. Results show that the proposed LSTM-NN method can significantly enhance the decoding of multiple intensity level rolling shuttered pattern. The proposed scheme also allows a large tilting angle of ±70° between the transmitter (Tx) and receiver (Rx).

The Applications of Digital Signal Processing Techniques for Enhancing the Performance of High Speed Optical Communication Systems

This paper presents Improving performance of 112 Gbits optical coherent communications systems using digital signal processing techniques to compensate linear impairments that are exposed to the signal during its propagation in optical fiber. In this paper, different multi-level modulation formats (DP-QPSK, DP 8PSK, DP- 16PSK and DP-16-QAM) were compared for the same data rate at different distances from the transmitter without amplification. the comparison was done by measuring bit error rate value. We used digital signal processing techniques for linear effects compensation. Where chromatic dispersion was compensated using a simple digital filter, and Polarization mod dispersion was realized by applying the constant-modulus algorithm (CMA). The phase and frequency mismatch between the transmitter and the local oscillator in the receiver was compensated using the modified Viterbi-Viterbi algorithm. We used the Optical simulator Opti-system , which was linked with MATLAB R2011 in order to implementation DSP algorithms and analyze obtained results.

Impact of Environmental Influences on Multilevel Modulation Formats at the Signal Transmission in the Optical Transmission Medium

This paper is devoted to the analysis of environmental influences in the optical transmission medium and their impacts on multilevel modulation formats. An attention is focused on main features and characteristics of environmental negative influences of optical fibers. Consequently, principles for appropriate multilevel modulation formats are introduced together with block schemes representing their main functionalities. The created Simulink model for technologies and communications is verified for real conditions in the optical transmission medium. It can allow executing requested analysis for environmental influences on advanced multilevel modulation formats at the signal transmission. Finally, a comparison of considered multilevel modulations is introduced, using constellation diagrams, signal characteristics, eye diagrams and waterfall curves of individual signals.

PAM4 rolling-shutter demodulation using a pixel-per-symbol labeling neural network for optical camera communications.

The typical optical camera communication (OCC) modulation scheme is based on binary intensity modulation. To increase the transmission data rate, multi-level modulation format is highly desirable. In this work, we bring forward and demonstrate a rolling shutter 4-level pulse amplitude modulation (PAM4) demodulation scheme for OCC systems using pixel-per-symbol labeling neural network (PPSL-NN) for the first time up to the authors' knowledge. A bit-rate distance product of 28.8 kbit/s • m per color is achieved. The proposed scheme is to calculate and re-sample the pixel-per-symbol (PPS) to make sure the same number of pixels in each PAM4 symbol is corresponding to a label for the neural network. Experiment results reveal that the proposed scheme can efficiently demodulate high speed PAM4 signal in the rolling shutter OCC pattern.

High-power Si-Ge photodiode assisted by doping regulation.

High-power silicon-based photodiodes are key components in many silicon photonics systems, such as microwave photonics systems, an optical interconnection system with multi-level modulation formats, etc. Usually, the saturation power of the silicon-germanium (Si-Ge) photodiode is limited by the space-charge screening (SCS) effect and the feasibility of the fabrication process. Here, we propose a high saturation power Si-Ge photodiode assisted by doping regulation. Through alleviating the SCS effect of the photodiode, we successfully demonstrate an 85.7% improvement on the saturation power and a 57% improvement on the -1 dB compression photocurrent. The proposed high-power Si-Ge photodiode requires no specific fabrication process and will promote the low-cost integrated silicon photonics systems for more applications.

Reduced-state MLSE for an IM/DD system using PAM modulation.

The performance of a high-speed intensity-modulation (IM)/direct-detection (DD) transmission system could be limited by the bandwidth of optical transceivers. One popular way to cope with this performance limitation is to utilize the maximum likelihood sequence estimation (MLSE) at the receiver. However, a practical problem of MLSE is its high implementation complexity. Even though the channel impulse response can be truncated by using a two-tap filter before applying the MLSE, it still faces an implementation problem when used for multi-level modulation formats. In this paper, we propose and demonstrate a reduced-state MLSE for band-limited IM/DD transmission systems using M-ary pulse amplitude modulation (PAM-M) formats. We use a conventional Viterbi algorithm to search a reduced-state trellis, which is constructed by using the coarse pre-decision of the signal equalized by a feed-forward equalizer. Thus, the proposed MLSE reduces the implementation complexity significantly. We evaluate the performance of the proposed reduced-state MLSE over 100∼140-Gb/s PAM-4/6/8 transmission systems implemented by using a 1.3-µm directly modulated laser. The results show that the proposed MLSE achieves almost the same performance as the conventional MLSE but reduces the implementation complexity by a factor of 4∼10 when the complexity is assessed by the number of multiplications and additions.

Effects of all-optical wavelength conversion on signal quality under variable-channel spacing in elastic optical networks

All-optical wavelength converters (AOWCs) based on parametric amplification in highly nonlinear fibers are investigated and evaluated, aiming at application flex-grid optical networks. A multichannel transmission system with a multilevel modulation format at 56-Gb / s bit rate is adopted. Parameters such as symbol error rate and signal bandwidth allocation are evaluated considering the optical signal-to-noise ratio, the power reception level, the channel spacing, and the impact of several cascade conversions—series of multiple AOWCs—on the signal quality. This work was carried out with an analytical modeling and computer simulations. Results reveal consistent modeling and adequate operation of the proposed AOWCs for flex-grid wavelength-division multiplexing networks and indicate that the number of conversions is limited by the size of the converter operating band and the allowed flex-grid slot size.

Principle of Coherence Optical Systems-Current Applications and Future Challenges

This paper reviews the concept of coherent optical systems as future digital optical communication systems having capacity of achieving 100Gbps speed. While the transmission capacity increases in wavelength-division multiplexed (WDM) system, coherent technologies have been in large interest in recent years. The interest lies in finding methods of increasing the bandwidth demand with multilevel modulation formats based on coherent technologies. We also discuss about requirements and challenges in implementation of respective digital receivers and associated signal processing.

Pure phase modulation based on a silicon plasma dispersion modulator.

Plasma dispersion modulators (PDMs), such as carrier injection modulators and carrier depletion modulators, are widely used for high speed phase modulation in silicon photonic circuits, but they suffer from spurious intensity modulation. This can be a problem in coherent communication systems that make use of complex multi-level quadrature modulation formats, as well as analog applications such as microwave photonics. In this article, a method to achieve pure phase modulation using PDMs is proposed based on a configurable modulator circuit. The configurable modulator is implemented as a Mach-Zehnder interferometer with a PDM and tunable couplers (TCs). The spurious intensity modulation of the phase modulated lightwave can be compensated by tuning the coupling ratios of the TCs and the phase delay between the two arms of the MZI. Simulation results show that for a depletion modulator, the 1.26 dB spurious intensity modulation can be suppressed down to 0.023 dB within a phase range of 0.4π, and for injection modulator, the 1.27 dB spurious intensity modulation can be suppressed down to 0.07 dB within a phase range of 0.76π.

Examination of Potential Terrestrial System Effects From Raman Pumps Below Cable Cutoff in G.654.E Fibers

ITU G.654.E was recently approved as a new optical fiber standard for terrestrial systems. While G.654-compliant fibers have been used extensively in undersea submarine networks for some time, the G.654.E standard is optimized for terrestrial systems and, in particular, it enables the use of large effective area fibers to extend the reach of new high data rate signals with multilevel modulation formats. The new G.654.E fiber standard increases the maximum cable cutoff wavelength specification to the lower edge of the C- band, 1530 nm. We analyze potential signal impairments that may arise from Raman amplification with pump wavelengths below cable cutoff. We study the case where Raman pump relative intensity noise (RIN) is caused only by Raman pump multipath interference and show that its effect is negligible compared to the contribution from inherent Raman pump RIN that is constant along a fiber propagation length. We find negligible effects are expected even under worst-case condition.

Semi-analytical method for designing finite sets of discriminated solitons and soliton molecules in fibres

ABSTRACTAn infinite number of solitons, as well as soliton molecules belonging different families, are allowed to propagate stably along a given dispersion-managed optical fibre. In this paper, we propose a semi-analytical method to systematically find a finite number of solitons and two-soliton molecules. Determined solitons and soliton molecules present a greatest power gap between them, to make them sufficiently distinct symbols for encoding several bits of information per symbol in a multilevel modulation format in fibre-optic transmission systems. Furthermore, for illustrative purposes, we discuss the possibility of encoding three bits per symbol.

Recent Progress with Next Generation High-Speed Ethernet Optical Device Technology

Ethernet has become an indispensable technology for communications, and has come into use for many applications. At the IEEE, high-speed standardization has been discussed and has seen the adoption of new technologies such as multi-level modulation formats, high baud rate modulation and dense wave length division multiplexing. The MSA transceiver form factor has also been discussed following IEEE standardization. Optical devices such as TOSA and ROSA have been required to become more compact and higher-speed, because each transceiver form factor has to be miniaturized for high-density construction. We introduce the technologies for realizing 100GbE and those applicable to 400GbE. We also discuss future packages for optical devices. There are many similarities between optical device packages and electrical device packages, and we predict that optical device packages will follow the trends seen in electrical devices. But there are also differences between optical and electrical devices. It is necessary to utilize new technology for specific optical issues to employ advanced electrical packaging and catch up the trends.

A survey on key roles of optical switching and labeling technologies on big data traffic of Data Centers and HPC environments

The tremendous big data and IP traffic growth rate between interconnected Data Centers (DC) and High Performance Computing (HPC) environments have imposed the need for ultrahigh link capacities and ultrahigh packet switching speeds, at network nodes. In order to overcome these ultrahigh demands, and particularly packet routing and forwarding speeds, long tested and established technologies, such as optical switching and labeling technology, seem to provide adequate solutions, not only by conveying ultrahigh bit rate data streams, but also by achieving multi Tb/s cross connection throughputs, in a cost and energy efficient way. By adoption of optical switching and labeling technology, big data streams are propagating directly in optical layer, thus lessening down bottlenecks, latency issues, and multi stage hierarchy layering. This paper, apart from optical switching and labeling potentials, investigates thoroughly other critical issues, strictly related to the proper choice of employing a switching architecture layout, such as its implementation technology, its elasticity potentials, in terms of flexible bandwidth (BW) provisioning, its adopted control plane lying on top of data infrastructure plane, providing cognition, control and orchestration over its network elements, as well as related to the proper choice of optical labeling techniques adopted, in conjunction with current, advanced, coherent, multi level modulation formats, for ultrahigh link capacities and packet switching speed demands of scalable, big data interconnected DCs and HPC environments.

Transmission characteristics of vortex beams in a sixfold photonic quasi-crystal fiber

In an optical fiber communication system, vortex beams have aroused great interest in the last several decades. Vortex beams possess many intriguing properties. For example, they have the ability to carry orbital angular momentum (OAM) which is mutually orthogonal. The OAM is a fundamental physical quantity of light which can be used as information carriers for transmission channel of optical fiber. Combined with the existing multiplexing techniques such as wavelength division multiplexing technique, advanced multilevel amplitude modulation formats, etc., the vortex beams provide an alternative to the increase of the transmission capacity and spectral efficiency of the optical fiber transmission system. Recently, long-length transmission of vortex-beam in optical fiber has been realized and there have also occurred some new designs of optical fiber on vortex beams, such as air-core ring shaped fiber, graded index vortex fiber, multi-ring fiber, and supermode fiber. Photonic crystal fiber (PCF) is flexible in design. Therefore, it is easy to regulate the transmission performance of PCF by adjusting the radius and the pitch of the air holes and so on. In this paper, we propose a newly designed sixfold photonic quasi-crystal fiber (SPQCF) to transmit vortex beams stably. Transmission characteristics of this newly designed fiber are simulated and calculated by using COMSOL multiphysics software. When the wavelength of the incident light is 1550 nm, the effective index difference between the vortex modes in a group is more than 10<sup>–4</sup> which is large enough to preclude the LP modes from being formed, and to transmit 7 vector modes (10 OAM modes). Changing the radius and pitch of the air holes, we can regulate the dispersion characteristic and confinement loss of the SPQCF flexibly. At 1550 nm, the confinement loss of the SPQCF maintains 10<sup>–8</sup>−10<sup>–7</sup> which is low enough to confine the vortex beams in the fiber core. When the incident light wavelength of HE<sub>21</sub> ranges from 1500 nm to 1800 nm (<i>r</i><sub>0</sub> = 1.9 μm), the dispersion coefficient of the SPQCF is between 63.51−65.42 ps·nm<sup>–1</sup>·km<sup>–1</sup> which tends to be flat. By changing <i>r</i><sub>0</sub>, the flat trend is adjusted to different wavelength range. This dispersion characteristic possesses great potential for the transmission of optical solitons. The effective mode area (HE<sub>21</sub>) is about 40 μm<sup>2</sup> and the nonlinear coefficient (HE<sub>21</sub>) is maintained on the order of 10<sup>–3</sup> between 1500−1600 nm. These features suppress the generation of nonlinear effect in the fiber and benefit the transmission of vortex beams. The stable transmission distance is longer than 1 km. In summary, we design a new type of PCF featuring quasi-crystal structure which has a ring shaped fiber core and supports the transmission of vortex beams stably.

TDM-PON PAM Downstream Transmission for 25 Gbit/s and Beyond

The optical access network is currently driving studies on transmissions beyond 10 Gbit/s. This paper reports an analysis of Pulse Amplitude Modulation (PAM), seen as a promising candidate for future Passive Optical Networks (PON). Previous 25 Gbit/s real-time PAM4 results are extrapolated here with simulations to higher bit rates and a higher number of PAM levels. Our main goal is to evaluate the compliancy of PAM with the existing standards and legacy networks as far as fiber length, optical budget class, and wavelength plan are concerned. The simulations enlighten us as to the challenges of multilevel modulation formats, such as noise and jitter, compared to the currently adopted Non-Return-to-Zero (NRZ).