Direct Modulation Scheme Research Articles

Suppression of Nonlinear Optical Effects in DWDM-PON by Frequency Modulation Non-Coherent Detection

We propose a simple and cost-effective method, using a direct frequency modulation (FM) and noncoherent detection (NCD) scheme, to suppress the nonlinear optical effects in dense wavelength division multiplexed (DWDM) optical communication. The FM transmitter comprises a directly modulated distributed feedback laser and a saturable semiconductor optical amplifier. In the NCD receiver, an optical slope filter as the FM to intensity modulation (IM) signal convertor is placed before a conventional photodetector. Because the FM signal has more evenly distributed optical power, bit-pattern-dependent nonlinear effects are consequently suppressed. After analyzing the nonlinear effects in the FM-NCD system and traditional IM direct detection (IM-DD) system, we found that the minimum achievable BER of the proposed FM-NCD scheme is 40 dB smaller. Moreover, a 2 Tbps (10 Gb/s × 200 channels) capacity was achieved by the FM-NCD system in 100 km DWDM passive optical networks (PONs), which is twice the capacity of the IM-DD system (10 Gb/s × 100 channels) under the same condition. These results indicate that WDM-PONs with the cost-effective FM-NCD scheme are strong candidates for future broad access networks and show great potential for the combination of optical access and metro networks for future generations of PONs.

Compact single-seed, module-based laser system on a transportable high-precision atomic gravimeter

A single-seed, module-based compact laser system is demonstrated on a transportable 87Rb-based high-precision atomic gravimeter. All the required laser frequencies for the atom interferometry are provided by free-space acousto-optic modulators (AOMs) and resonant electro-optic phase modulators (EOMs). The optical phase-locked loop between the two optical paths derived from the same laser provides an easy frequency manipulation between two laser frequencies separated by the hyperfine frequency of 6.835 GHz using an AOM and an EOM, respectively. Our scheme avoids parasite Raman transitions present in the direct EOM modulation scheme (modulating directly at the frequency of the hyperfine splitting), which have detrimental effects on the accuracy of the gravity measurements. The optical phase-locked loop also provides a convenient way for vibration compensation through the Raman lasers' phase offset. Furthermore, the modular design approach allows plug-and-play nature on each individual optic module and also increases the mechanical stability of the optical systems. We demonstrate high-precision gravity measurements with 17.8 μGal stability over 250 s averaging time and 2.5 μGal stability over 2 h averaging time.

A Direct Carrier-Based Modulation Scheme With Full Index Range for DC-Link Current Ripple Mitigation of a Current Source Converter

This article presents a direct carrier-based modulation scheme to mitigate the dc-link current ripple for a current source converter (CSC). Excessive dc-link current ripple increases system losses, causes current distortion, and leads to more electromagnetic interference, so, it should be mitigated. Compared with increasing dc-link inductor or switching frequency, it is more attractive to mitigate dc-link current ripple with modulation schemes. However, existing modulation schemes suffer from either high dc-link current ripple or shrink of modulation index range. To address this issue, this article analyzes the mechanism of mitigating the dc-link current ripple and proposes a new modulation scheme for a CSC. By employing two kinds of non-nearest three vectors to synthesize the reference current, the proposed scheme successfully extends the index range from [ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sqrt 3 /3$</tex-math></inline-formula> , 1] to <xref ref-type="bibr" rid="ref1" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">[0, 1]</xref> . For easier implementation, its carrier-based modulation, which can automatically select the desired vectors and last predetermined dwell times, is also presented. Theoretical analyses show that the proposed scheme can mitigate the dc-link current ripple over the full index range for a CSC. The effectiveness of the proposed modulation scheme is verified experimentally with a CSC operating as a rectifier.

A Mechanical Antenna With Frequency Multiplication and Phase Modulation Capability

This article reports on a novel direct frequency multiplication and phase modulation (PM) scheme for mechanical antennas operating at the very-low-frequency band (3-30 kHz) and lower. This antenna is intended as a transmitter for underwater and underground communication applications. In mechanical antennas, instead of using signal generators to drive the antenna for radiation, the radiation is induced by accelerating (rotating) electric charges or permanent magnets. One important challenge for mechanical antennas is to find an approach to modulate the information signal with sufficient bandwidth. We propose a novel PM scheme by rotating a permanent magnet which is sandwiched between two pairs of orthogonal bow-tie shape high- μ magnetic material plates. By moving the angular position of the magnetic plates, it is shown that the phase and amplitude of the EM signal can be modulated. The amplitude modulation is significant and this provides an additional important feature, which allows the antenna to generate higher harmonics of the rotation frequency, and thereby it facilitates radiation at a much higher frequency than the rotational frequency of the magnet. Hence, higher range and more radiated power can be achieved. Here, the performance of proposed mechanical antenna with its PM and frequency multiplication capability is first verified analytically and through numerical simulations using ANSYS Maxwell. Then, a prototype of the antenna is fabricated with the aid of low cost 3-D-printing technology and its performance is measured. Measurements are in good agreement with the simulation predictions.

A Simplified Radio-Over-Fiber System for Over 100-km Long-Reach n-QAM Transmission

A simplified and cost-effective architecture is proposed and experimentally demonstrated with a millimeter wave (MMW) signal over fiber and free-space wireless transmission. The distributed feedback (DFB) laser is operated in optimized settings after measuring its characteristics and thus the 6 Gbps 64–quadrature amplitude modulation (64-QAM) signal can be successfully transmitted after 20- and 50-km fiber and 1-meter wireless transmission with EVM of 5.98% and 6.87%, respectively. Moreover, the EVM of 10.8% over 100-km long reach is also achieved with the 32-QAM modulation format and data rate of 5 Gbps. The experimental results show that the proposed direct modulation scheme can satisfy applications in metro and rural areas.

An Energy-Synchronous Direct Antenna Modulation Method for Phase Shift Keying

A novel scheme for transmitting broadband phase shift keyed signals from electrically small antennas using energy-synchronous direct antenna modulation is described. We outline fundamental operating principles of the method and experimentally compare its performance to that of a conventional band-limited transmit antenna with the same electrical size and radiation efficiency. Transmitted waveforms are analyzed in the time domain both at RF and baseband. Results show significant increases in signal quality, suggesting a larger effective transmit bandwidth and greater potential throughput when the proposed direct antenna modulation scheme is used.

Analysis and Design of High-Order QAM Direct-Modulation Transmitter for High-Speed Point-to-Point mm-Wave Wireless Links

A novel high-speed wireless transmitter (TX) architecture is presented that directly transforms incoming data bits into high-order $4^{M}$ -quadrature amplitude modulation (QAM) constellation by adding multiple quadrature phase shift keying (QPSK) signals with appropriate amplitude ratios. The costly high-speed digital-to-analog converters (DACs) in conventional TXs are thus completely avoided, resulting in a highly integrated solution amenable to ultra-high speeds and operating frequencies. Design tradeoffs are analyzed in detail. Based on this article, a TX prototype at 115-GHz carrier frequency implementing the 16QAM direct-modulation scheme is fabricated in a 180-nm SiGe BiCMOS process ( $f_{\text {MAX}} = 270$ GHz). Wireless testing at a 20-cm distance with 25-dBi horn antennas on both transmitting and receiving side measures 20-Gb/s data rate with an error vector magnitude (EVM) of −15.8 dB and modulated output power of +1 dBm. The TX consumes 520 mW of power and occupies 3.17 mm2 of active area.

First demonstration of Brillouin optical correlation-domain reflectometry based on external modulation scheme

We demonstrate basic operation of Brillouin optical correlation-domain reflectometry (BOCDR) based on an external modulation scheme, which can mitigate three demerits of a conventional direct modulation scheme. A 0.8 m long strain in a 2.8 m long silica fiber is successfully detected in a distributed manner. This scheme can be employed not only in BOCDR but also in Brillouin optical correlation-domain analysis.

Performance Analysis of Single Side Band Orthogonal Frequency Division Multiplexing in Radio over Fiber System with Direct and External Modulation Schemes

Abstract Radio over fiber is a technology in which the light signals at the central station are modulated by a radio frequency signal, and by using the optical fiber it is transmitted to base stations. The data stream being sent is separated into subcarrier frequencies by the technique of orthogonal frequency division multiplexing. The orthogonality between the sub-carriers is provided by FFT modulation, which avoids the demodulators from accepting other frequencies. It is possible to transmit high-data rate signals over short and long distance by implementing orthogonal frequency division multiplexing with radio over fiber system along with the optical fiber. This enhances the system with more coverage area, and without increasing the cost of the system it provides the flexibility. The proposed work is to investigate the performance analysis of single side band orthogonal frequency division multiplexing in radio over fiber system with external and direct modulations using spectral analysers and constellation visualizers. The Mach-Zender modulator is proposed for external modulation, the vertical cavity surface emitting laser is used in direct modulation. The simulations were carried out by using Optisystem Software version 15.0 with global parameters for both intensity modulation schemes.

An energy-efficient and ultra-low-voltage power oscillator in CMOS 65 nm

In a direct modulation scheme and particularly in portable and wireless sensor network applications, the oscillator limits the transmitter efficiency therefore, the oscillator efficiency is critical and high-efficiency and high-power oscillators are increasingly required. This paper presents a high-efficiency and ultra-low-voltage power oscillator that operates at 2.4 GHz and is implemented in 65 nm CMOS technology. The power oscillator is a self-oscillating class-E power amplifier (PA) that utilizes a positive feedback system. A class-E PA is selected due to its high efficiency. The power oscillator has an optimized power consumption and output power, where a 2.4 mW power consumption is achieved under a 0.4 V power supply. The proposed oscillator demonstrates a maximum output power of $$-0.45$$ dBm and a peak efficiency of 37.5%. The oscillator is tunable between 1.66 and 2.78 GHz. The oscillator is robust to a $$\pm\,15\%$$ frequency deviation from a 2.44 GHz nominal frequency due to process, voltage, temperature variation.

Secure Optical QAM Transmission Using Chaos Message Masking

Abstract This work presents the joint use of advance modulation scheme i. e. m-QAM and optical chaos to combine the best of the two advantages i. e. higher data rates and security. A semiconductor laser diode is driven into chaotic region using direct modulation scheme and 4-QAM signal is added by Chaos message masking (CMS). The chaotically masked data stream is transmitted over an optical communication link to investigate the propagation issues and synchronization of chaos at the receiver. The transmitted chaos is synchronized at the receiver to unmask the QAM stream and binary data by using subtraction rule and conventional QAM demodulator, respectively. The deterioration of constellation diagrams and bit error rate found dependent upon transmitter/receiver synchronization and link parameters. The use of 4-QAM chaotic scheme is extendable to m-QAM and is applicable to long haul, short haul point to point and for passive optical networks.

Proposal of external modulation scheme for fiber-optic correlation-domain distributed sensing

All the previous configurations of optical correlation-domain reflectometry (OCDR) have been based on the direct modulation of laser driving current. However, some lasers are not designed for high-speed large-amplitude modulation, and the modulation amplitude and frequency are mutually dependent. In this work, to mitigate these drawbacks, we propose an external modulation scheme for OCDR. After confirming its basic operation, we show that a non-specially designed laser can be employed. We also clarify that the interdependence of the modulation amplitude and frequency is negligibly small in this scheme, which is not the case for the conventional direct modulation scheme.

Design Issues of Digital and Analog Chaotic RoF Link Using Chaos Message Masking

This work presents the joint use of Radio Over Fiber and optical chaos to investigate the secure ROF link. Merging the two technologies, optical chaos for physical layer communication security and Radio over Fiber creates new design issues which have been identified and studied in detail in this paper for both analog Radio Frequency/Intermediate Frequency and digitized data. A semiconductor laser diode is driven into chaotic region using direct modulation scheme and RoF signal is added by chaos message masking scheme. The chaotically masked signal is transmitted over an optical communication link to investigate the propagation issues and synchronization of chaos at the receiver. The transmitted chaos is synchronized at the receiver to unmask the signal by using subtraction rule. To investigate the performance of chaotic communication system for Radio over Fiber transmission, the figure of merits like Bit error rate, Quality factor, Eye Opening Penalty and Root-mean-squared phase jitter are studied for digital data and Signal to Noise ratio and Total Harmonic Distortion are studied for analog waveform to address the effects of link length and data rate/message bandwidth.

24–26 GHz radio-over-fiber and free-space optics for fifth-generation systems

This Letter outlines radio-over-fiber combined with radio-over-free-space optics (RoFSO) and radio frequency free-space transmission, which is of particular relevance for fifth-generation networks. Here, the frequency band of 24-26GHz is adopted to demonstrate a low-cost, compact, and high-energy-efficient solution based on the direct intensity modulation and direct detection scheme. For our proof-of-concept demonstration, we use 64 quadrature amplitude modulation with a 100MHz bandwidth. We assess the link performance by exposing the RoFSO section to atmospheric turbulence conditions. Further, we show that the measured minimum error vector magnitude (EVM) is 4.7% and also verify that the proposed system with the free-space-optics link span of 100m under strong turbulence can deliver an acceptable EVM of <9% with signal-to-noise ratio levels of 22dB and 10dB with and without turbulence, respectively.

Filter Bank Multicarrier (FBMC) for long-reach intensity modulated optical access networks

Filter Bank Multi Carrier (FBMC) is a modulation scheme which has recently attracted significant interest in both wireless and optical communications. The interest in optical communications arises due to FBMC's capability to operate without a Cyclic Prefix (CP) and its high resilience to synchronisation errors. However, the operation of FBMC in optical access networks has not been extensively studied either in downstream or upstream. In this work we use experimental work to investigate the operation of FBMC in intensity modulated Passive Optical Networks (PONs) employing direct detection in conjunction with both direct and external modulation schemes. The data rates and propagation lengths employed here vary from 8.4 to 14.8Gb/s and 0–75km. The results suggest that by using FBMC it is possible to accomplish CP-Less transmission up to 75km of SSMF in passive links using cost effective intensity modulation and detection schemes.

Affordable dispersion mitigation with an analog electrical filter.

A new method for dispersion mitigation is presented for low-cost and simple networks. The method does not require dispersion-compensating fibers, special optical filters, coherent detection, or external modulation. It can work in a direct modulation scheme and with a standard optical detector. The disadvantage of this method is the requirement to operate in the weak modulation regime of the signal. In this regime, the dispersive channel can be regarded as linear in the power domain, and not only in the field domain, so that the effects of dispersion can be reduced with a proper electronic filter. Since electronic filters are usually considerably cheaper than coherent optical solutions, this solution can be implemented in low-cost networks, where dispersion is a more severe problem than noise. We show that by adding the filter to a low-noise on-off keying (OOK) system it is possible to transmit data at bit rates of 50 Gb/s to distances at least sixfold larger than its OOK limit (6 km in this case), i.e., 40 km and beyond.

Detection of a directly modulated terahertz light with quantum-well photodetector

We demonstrate a wireless transmission link at 3.9 THz over a distance of 0.5 m by employing a terahertz (Hz) quantum-cascade laser (QCL) and a THz quantum-well photodetector (QWP). We make direct voltage modulation of the THz QCL and use a spectral-matched THz QWP to detect the modulated THz light from the laser. The small signal model and a direct voltage modulation scheme of the laser are presented. A square wave up to 30 MHz is added to the laser and detected by the THz detector. The bandwidth limit of the wireless link is also discussed.

Direct Modulation of Lanthanide Emission at Sub-Lifetime Scales

The long lifetime of lanthanide emitters can present a challenge for conventional pump-based modulation schemes, where the maximum switching speed is limited by the decay time of the excited state. However, spontaneous emission can also be controlled through the local optical environment. Here, we demonstrate a direct modulation scheme enabled by dynamic control of the local density of optical states (LDOS). Specifically, we exploit the LDOS differences between electric and magnetic dipole transitions near a metal mirror and demonstrate that rapid nanometer-scale mirror displacements can modulate the emission spectra of trivalent europium ions within their excited state lifetime. The dynamic LDOS modulation presented here can be readily extended to faster optical modulation schemes and applied to other long-lived emitters to control the direction, polarization, and spectrum of spontaneous emission at sublifetime scales.

Pseudo-Random Binary Sequence Selection for Delay and Add Direct Sequence Spread Spectrum Modulation Scheme

Recently, correlation delay shift keying (CDSK) has been proposed as a modulation scheme for noncoherent detection which inherits the advantages of conventional spread-spectrum communications using a chaotic reference signal. The CDSK modulation scheme transmits the sum of a chaotic reference signal with its delayed modulated version. In this paper, we consider the utilisation of pseudo-random binary sequences (PRBS) instead of chaotic reference signals. This new modulation scheme is defined as the delay and add direct sequence (DADS) modulation scheme. DADS offers some interesting properties compared to CDSK, mainly the possibility to select some PRBS which improves the BER performance. To this end, a new criterion for PRBS selection is proposed. Theoretical analysis and simulation results show that the BER performance of CDSK is comparable to DADS with arbitrary PRBS, however the DADS performance can be improved by 3 dB with PRBS selection.

A co-operative beamforming solution for eliminating multi-hop communications in wireless sensor networks

This paper proposes a new system architecture for continuous monitoring wireless sensor networks, utilizing recent developments in the areas of cooperative beam-forming, radio frequency integrated circuit technology, and spread-spectrum techniques. In the proposed system, nodes are activated only when they are triggered to send their own data to the fusion center directly, without burdening the network with MAC layer collisions and energy-consuming routing algorithms, thus guaranteeing long lifetime cycles. The nodes consist of a simple active reflector circuit which is synchronized by an incident wave transmitted by a distant base station. Since the nodes can be deployed in large numbers, the network acts as a single antenna array with high directive gain pointing towards the direction of the fusion center. At the receiver, data from each node can be de-multiplexed with the use of a frequency shift keying direct sequence spread spectrum modulation scheme that enables multiple access without destroying the beamforming characteristics of the network.