Modern Optical Communication Systems Research Articles

High-speed electro-optic modulator based on silicon nitride loaded lithium niobate on an insulator platform.

Electro-optic (EO) modulators, which convert signals from the electrical to optical domain plays a key role in modern optical communication systems. Lithium niobate on insulator (LNOI) technology has emerged as a competitive solution to realize high-performance integrated EO modulators. In this Letter, we design and experimentally demonstrate a Mach-Zehnder interferometer-based modulator on a silicon nitride loaded LNOI platform, which not only takes full advantage of the excellent EO effect of LiNbO3, but also avoids the direct etching of LiNbO3 thin film. The measured half-wave voltage length product of the fabricated modulator is 2.24 V·cm, and the extinction ratio is ∼20dB. Moreover, the 3 dB EO bandwidth is ∼30GHz, while the modulated data rate for on-off key signals can reach up to 80 Gbps.

Simulation of optical triode based on PT symmetric structure under electro-optic modulation

In modern microwave and optical communication systems, it is urgent to develop optical components with the same performance as electronic components. In this paper, a PT-symmetric structure including electro-optical medium is designed to construct optical triode. It is found that the structure has a pole effect of amplification under certain parameters. The frequency of the incident wave at the pole is taken as the carrier frequency, and the electrical signal is modulated into an optical signal. By setting the bias electric field, the modulated optical signal can be amplified in phase and out phase, and even by doubling frequency, and a new optical triode model is obtained. The designed model will play an important role in the design of optical integrated devices.

Single-pixel optical modulation analyzer: a low-complexity frequency-dependent IQ imbalance monitor based on direct detection with phase retrieval.

Tiny mismatches in timing, phase, and/or amplitude between in-phase (I) and quadrature (Q) tributaries in an electro-optic IQ modulator, namely IQ imbalance, can severely affect high baud-rate and/or high modulation-order signals in modern coherent optical communications systems. To maintain such analog impairment within the tight penalty limit over wavelength and temperature during the product lifetime, in-service in-field monitoring and calibration of the IQ imbalance, including its frequency dependence, become increasingly important. In this study, we propose a low-complexity IQ monitoring technique based on direct detection with phase retrieval called a single-pixel optical modulation analyzer (SP-OMA). By reconstructing the optical phase information lost during the detection process computationally via phase retrieval, SP-OMA facilitates the in-service in-field monitoring of the frequency-dependent imbalance profile without sending dedicated pilot tones and regardless of any receiver/monitor-side IQ imbalance. The feasibility of SP-OMA is demonstrated both numerically and experimentally with a 63.25-Gbaud 16QAM signal.

Spatial Jitter Influence on the Average BLER Performance of SIMO FSO Links over Atmospheric Turbulence Channels

In the recent years, Free Space Optics (FSO) technology has attracted significant research and commercial interest mostly because of its many advantages in comparison with other radio systems used for point-to-point connections. However, the reliable operation of these systems significantly depends on the conditions of the atmosphere in the area in which the optical beam propagates. The most important of these conditions are atmospheric turbulence and the misalignment between the optical beam and the receiver, which is also known as the pointing errors effect. In this work, in order to obviate the performance mitigation caused by these phenomena, we examined the most widely accepted and one of the most effective techniques, i.e., the implementation of receivers’ diversity. Various metrics have been investigated to evaluate the performance of such systems, but most of them do not take into account that the ultra-fast modern optical communication systems use blocks of bits for the transmission and codes for the detection and/or correction of erroneous bits. Thus, by taking these aspects into account, in this work, we investigated the combined impact of spatial jitter and atmospheric turbulence on the total average block error rate of an optical wireless system with receivers’ diversity. Novel closed-form analytical formulas were derived.

Complex vector fitting toolbox: a software package for the modelling and simulation of general linear and passive baseband systems

Complex vector fitting (CVF) is a robust method to efficiently and accurately model general linear and passive baseband systems. Such models are important for the analysis and design of modern electronic and optical communication systems. This letter introduces an efficient toolbox implementation of the CVF algorithm. The effectiveness of the code is illustrated on a suitable application example.

Ultra-Compact Broadband In-Line Mode Converter Based on a Width-Modulated Silicon Waveguide

Mode division multiplexing (MDM) technology is becoming increasingly important for modern optical communication systems. Here, an ultra-compact broadband in-line mode converter for quasi-TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">00</sub> and quasi-TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sub> on the silicon-on-insulator platform is proposed and demonstrated experimentally. In our device, the mode-conversion region consists of a continuously width-modulated waveguide with a footprint size as small as 1.32 × 4.52 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Its modulation profile is designed by using the particle swarm optimization algorithm. This device has a simulated conversion efficiency of about -0.174 dB and an insertion loss less than 0.153 dB within 100-nm wavelength bandwidth from 1500 nm to 1600 nm. Our design exhibits a favorable fabrication error tolerance and the fabricated device has achieved nearly the same conversion efficiency as the simulated one. Our concept can also be applied to design other high-performance mode converters, i.e., converting modes between quasi-TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sub> and quasi-TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">00</sub> . Our work suggests a very promising path for realizing compact integrated MDM systems.

IM/DD WDM-PON Communication System Based on Optical Frequency Comb Generated in Silica Whispering Gallery Mode Resonator

This article reports an implementation of a microsphere-based optical frequency comb (OFC) generator for substitution of individual laser arrays and simulates wavelength division multiplexed passive optical network (WDM-PON) based on this OFC generator. Our proposed generator is built based on silica microsphere, barely studied for fiber optical communication systems, and is a promising solution with photonic integration capability. Kerr OFC as a multiple light source containing more than 20 spectral carriers in the fundamental mode family in the C-band and beyond is demonstrated experimentally. Four of these OFC generator carriers with the highest peak power are studied in simulated 4-channel 393 GHz spaced WDM-PON. Additionally, we show this OFC as a source of optical carriers capable of providing data transmission over most utilized fiber types in modern optical communication systems, namely, single-mode fiber (SMF), non-zero dispersion-shifted fiber (NZ-DSF), and cut-off shifted fiber (CSF). We show through the simulations that error-free data transmission is possible, providing a total transmission capacity of 40 Gbit/s by using four OFC generated carriers.

Data Transmission Based on Exact Inverse Periodic Nonlinear Fourier Transform, Part II: Waveform Design and Experiment

The nonlinear Fourier transform has the potential to overcome limits on performance and achievable data rates which arise in modern optical fiber communication systems when nonlinear interference is treated as noise. The periodic nonlinear Fourier transform (PNFT) has been much less investigated compared to its counterpart based on vanishing boundary conditions. In this paper, we design a first experiment based on the PNFT in which information is encoded in the invariant nonlinear main spectrum. To this end, we propose a method to construct a set of periodic waveforms each having the same fixed period, by employing the exact inverse PNFT algorithm developed in Part I. We demonstrate feasibility of the transmission scheme in experiment in good agreement with simulations and obtain a bit-error ratio of $10^{-3}$ over a distance of 2000 km. It is shown that the transmission reach is significantly longer than expected from a naive estimate based on group velocity dispersion and cyclic prefix length, which is explained through a dominating solitonic component in the transmitted waveform. Our constellation design can be generalized to an arbitrary number of nonlinear degrees of freedom.

Photon Subtraction-Induced Plug-and-Play Scheme for Enhancing Continuous-Variable Quantum Key Distribution with Discrete Modulation

Establishing high-rate secure communications is a potential application of continuous-variable quantum key distribution (CVQKD) but still challenging for the long-distance transmission technology compatible with modern optical communication systems. Here, we propose a photon subtraction-induced plug-and-play scheme for enhancing CVQKD with discrete-modulation (DM), avoiding the traditional loopholes opened by the transmission of local oscillator. A photon subtraction operation is involved in the plug-and-play scheme for detection while resisting the extra untrusted source noise of the DM-CVQKD system. We analyze the relationship between secret key rate, channel losses, and untrusted source noise. The simulation result shows that the photon-subtracted scheme enhances the performance in terms of the maximal transmission distance and make up for the deficiency of the original system effectively. Furthermore, we demonstrate the influence of finite-size effect on the secret key rate which is close to the practical implementation.

Joint Superchannel Digital Signal Processing for Effective Inter-Channel Interference Cancellation

Modern optical communication systems transmit multiple frequency channels, each operating very close to its theoretical limit. The total bandwidth can reach 10 THz limited by the optical amplifiers. Maximizing spectral efficiency, the throughput per bandwidth is thus crucial. Replacing independent lasers with an optical frequency comb can enable very dense packing by overcoming relative drifts. However, to date, interference from non-ideal spectral shaping prevents exploiting the full potential of frequency combs. Here, we demonstrate comb-enabled multi-channel digital signal processing, which overcomes these limitations. Each channel is detected using an independent coherent receiver and processed at two samples-per-symbol. By accounting for the unique comb stability and exploiting aliasing in the design of the dynamic equalizer, we show that the optimal spectral shape changes, resulting in a higher signal-to-noise ratio that pushes the optimal symbol rate towards and even above the channel spacing, resulting in the first example of frequency-domain super-Nyquist transmission with multi-channel detection for optical systems. The scheme is verified both in back-to-back configuration and in single span transmission of a 21 channel superchannel originating from a 25 GHz-spaced frequency comb. By jointly processing three wavelength channels at a time, we achieve spectral efficiency beyond what is possible with independent channels. At the same time, one significantly relaxes the hardware requirements on digital-to-analog resolution and bandwidth, as well as filter tap numbers. Our results show that comb-enabled multi-channel processing can overcome the limitations of classical dense wavelength division multiplexing systems, enabling tighter spacing to make better use of the available spectrum in optical communications.

Hard-Decision Coded Modulation for High-Throughput Short-Reach Optical Interconnect.

Coded modulation (CM), a combination of forward error correction (FEC) and high order modulation formats, has become a key part of modern optical communication systems. Designing CM schemes with strict complexity requirements for optical communications (e.g., data center interconnects) is still challenging mainly because of the expected low latency, low overhead, and the stringent high data rate requirements. In this paper, we propose a CM scheme with bit-wise hard-decision FEC and geometric shaping. In particular, we propose to combine the recently introduced soft-aided bit-marking decoding algorithm for staircase codes (SCCs) with geometrically-shaped constellations. The main goal of this CM scheme is to jointly boost the coding gain and provide shaping gain, while keeping the complexity low. When compared to existing CM systems based on M-ary quadrature-amplitude modulation (MQAM, ) and conventional decoding of SCCs, the proposed scheme shows improvements of up to dB at a bit-error rate of in the additive white Gaussian noise channel. For a nonlinear optical fiber system, simulation results show up to reach increase. In addition, the proposed CM scheme enables rate adaptivity in single-wavelength systems, offering six different data rates between 450 Gbit/s and 666 Gbit/s.

Influence of Average Cavity Dispersion and Spectral Bandwidth on Passively Harmonic Mode Locked L-Band Er-Doped Fiber Laser

We systematically study the impact of average cavity dispersion and pulse spectral bandwidth on harmonic mode locking in the long-wavelength band from an Er-doped fiber laser using carbon nanotubes polyvinyl alcohol (CNTs-PVA) film mode locker. By carrying out pulse energy management through optimizing the average cavity dispersion and optical spectral bandwidth, 2.08 GHz repetition rate corresponding to 201st harmonic with 36.5 dB side mode suppression ratio (SMSR) can be realized under 205 mW pump power at 1597.53 nm. Furthermore, we systematically investigate the characteristics of the harmonically mode locked ultrashort optical pulses with different spectral bandwidths versus pump power under fixed cavity dispersion first. Key laser parameters have been studied including pumping efficiency, duration, energy, SMSR, and time bandwidth product. Enlightened by the experimental results, a simple law on preliminary and fast judgment of pulses frequency under fixed pump power is proposed, namely, by monitoring the pulse spectral bandwidth. The laser source featuring high repetition rate and L-band operation is greatly sought-after in some specific applications such as modern optical communication system etc.

A Universal Specification for Multicore Fiber Crosstalk

In order to transition multi-core fiber (MCF) technologies from their research state to volume production, the key design specifications have to be broadly agreed upon, first and foremost an acceptable level of inter-core crosstalk per unit length. Against common belief, we show that MCF crosstalk requirements per unit length are fairly independent of transmission distance in the context of modern coherent optical communication systems. As a consequence, a single value for the tolerable inter-core crosstalk per unit length can be used to specify MCFs, valid from metropolitan (~100 km) to trans-pacific (~10 000 km) deployment scenarios. The notion of a universal inter-core crosstalk specification allows for application-independent MCF designs (including a distance-independent optimum core density) and will facilitate the standardization and volume manufacturing of MCF.

Simulation of an inelastic dispersive phenomenon: stimulated Brillouin scattering in a single-mode fiber segment through parallelism

Stimulated Brillouin scattering (SBS) is one of the most important nonlinear phenomena because it limits the maximum transmission power in modern optical communication systems. Unfortunately, the simulation of SBS is time-consuming, since it requires estimating the solutions for a set of complex differential equations that describe this phenomenon. In this paper, a novel high-performance computing model intended to analyze the main dispersive effects present in modern fiber communication systems is proposed. A full optical characterization of the simulation results is included and compared with the efficiency and improved speed of our parallel implementation versus a previous sequential model for SBS. Also, an evaluation of the throughput of our parallel implementation using both central processing unit multi-core and graphics processing unit is presented. Results show that parallelism increases the performance of the simulation tenfold.

Asymmetric optical transmission through silver film with a hyperbolic air hole

Asymmetric optical transmission is one of the most important functionalities in modern optical communication systems. However, it has been impeded due to the low transmission ratio of forward transmittance to the backward transmission ratio. This paper is a continuation of the paper by Peng et al. [Opt. Express22, 17546 (2014)OPEXFF1094-408710.1364/OE.22.017546]. It was found that the guided mode reflects remarkably on the bottom of a conical air hole. A guided mode reaching the bottom of a conical air hole is the major factor leading to the asymmetric optical transmission effect. It reminded us that the asymmetric optical transmission effect of a structure having a much steeper inner wall would be better. Thus, we proposed silver film with a hyperbolic air hole (HAH), of which the transmission ratio can be as high as 17.5 at a proper wavelength. Through analysis, as we expected, the mechanism is the steep inner wall of the silver film HAH really can inhibit guided-mode reflection.

DEVICE FOR BINARY DATA TRANSMITTING AND RECEIVING OVER A FIBEROPTIC COMMUNICATION CHANNEL

When transferring data over optical fiber communication channels, it is required to provide data security and the authenticity of their source. To limit the access to the data for a third party, there can be applied quantum-cryptographical systems which are supposed to transfer every data bit by means of low power optical signals containing radiation photons the number of which could be in the range from 10 to 1, however, are far from being perfect and suffer from shortcomings, the main of which being a large number of errors due to the depolarization effect of optical radiation. The aim of this work was, therefore, to create device for sending and receiving confidential data which could provide complete security of transferred data, inherent to quantum-cryptographical communication systems at the same time could have a low number of such errors.A device for quantum-cryptographic communication system with a photon counter applied as a receiving module has been proposed. The possibility to use silicon avalanche photodetectors operating in the photon counting mode for confidential information transmission systems and defining authenticity of the source of transmitted information has been shown.I develop modern optical fiber communication system incorporating avalanche photodetector photon counter as a receiving module, that allow to detect unauthorized access to information and violation of its integrity and speed up the exchange of information in comparison with well-known quantum-cryptographical communication systems.

Enhancement the performance of an electro-optic switch by analysis the effect of tensile stress, axial and radial strain

This research presents a technique of an electro optic effect for suggestion model and it optimizes implementation of an electro optics switch using Mat lab simulation program (10). this technique includes design a mathematical model for analysis the effect of tensile stress(y), axial (z)and radial (x)strain on the performance evaluation of an electro optic switch also, it analysis an effect the change of length L and width w of arm of switch. Finally, an active switch optimizes, using the analytical model and considers important device in the modern optical communication system.

Estimation of analytical model for enhancement and implementation of an electro-optic switch

This research presents a technique of an electro optic effect for enhancement the an accomplishment of an electro optics switch using Mat lab simulation program . this technique includes design a mathematical model for evaluate the effect of different parameters such as refractive index (n), distance of separation between waveguides (d), length of electrodes (L), relative refractive index (Δn), and switching voltage (V), on the DC bias voltage of an electro optics switch. In this work the investigation of performance of an electro optics switch by analysis of an effect of distance between waveguides and the changing of refractive index on the bias voltage (V), and which optimizes when the wavelength is from 1300 into 1550 nm. Finally, an electro-optic active switch is designed and optimized, using the analytical model and which considers important device in the modern optical communication system.

Suppression of a Nonlinear Effect for High Data Transnission Rate Systems with a Wavelength Division Multiplexer Using the Optimization of Fiber Properties

The nonlinear crosstalk has a detrimental impact on the efficiency of optical communication systems. It becomes stronger with increasing the data transmission rate and transmission distances. We have investigated and estimated the behavior of a nonlinear effect such as the four wave mixing (FWM) under varying the fiber properties such as the frequency channel spacing, fiber cross-section area, and dispersion. In addition, the demeanour of FWM is observed with the use of two types of an advanced modulation format: Return-to-Zero-Frequency Shift Keying (RZ-FSK) and Non Return-to-Zero Frequency Shift Keying (NRZ-FSK). It is found that the FWM power obtained, when the frequency channel spacing is 80 GHz, is drastically reduced to –77dBm in the RZ-FSK scheme, i.e., a reduction ratio in FWM is more than 21%. The simulation also shows that the RZ-FSK modulation format under the channel spacing impact offers a better bit error rate (BER) than NRZ-FSK in modern optical communication systems.

Realization of mode independent multichannel transmission filter by controlling the photon localization in symmetric cavities

Modern optical communication system requires multiple channel transmission in different spectral band for different applications. Here we are proposing a microresonator structure for closely spaced, mode independent multiple channel transmission in 3rd transmission window by controlling the localization of photons in its cavity. To design the proposed structure, the spectral filtering properties of a single cavity planar microresonator are studied initially. In conventional multilayer dielectric filter, the localization of light in the microcavity is generally controlled by controlling the number of layers. Here we have shown that application of electric field across the structure, designed with suitable materials can also affect the localization of light in the cavity as well as the Q values of the transmission spectra. The mode independent characteristic of the structure is found from the study of its transmittance characteristics at different angle of incidence of light. It is found that the results of single cavity structure hold well for multicavity resonator structure also, in the regime of optical communication wavelength range. A method to increase the number of transmission channels is also proposed here keeping the fabrication perspective in mind.