Pulse Modulation Formats Research Articles

Spectrally efficient DWDM system using DQPSK modulation

Abstract DWDM technology is gaining popularity in fiber-optic communication due to high-speed transmission channel capacity demands. The adoption of this system has been driven by this demand. Researchers focus on optimizing data transmission parameters within channel characteristics for maximum performance. The signal’s form typically influences the optical channel capacity, the guiding medium’s dispersive and non-linear characteristics, and interference from multiple sources. This study investigates how efficient modulated systems control linear and non-linear impairments in optical channel capacity due to high data rates transmitted over optical communication networks, aiming to circumvent current issues. The selection of pulse shape and modulation format is crucial for an effective DWDM optical communication system due to the rigorous specifications of the optical channel. Modulation formats like intensity and phase have become viable network design technologies, allowing for network creation that operates to the designer’s satisfaction. This academic study examines the theoretical properties of various models used to analyze and recreate high-speed optical communication links, aiming to understand the impact of system components and modulation techniques.

Gerchberg-Saxton Based FIR Filter for Electronic Dispersion Compensation in IM/DD Transmission Part II: Experimental Demonstration and Analysis

In this article, the first experimental demonstration of a non-iterative electronic dispersion compensation (EDC) solution implemented at the transmitter using a finite impulse response (FIR) filter optimized with the Gerchberg-Saxton (GS) algorithm, is presented, for intensity-modulation and direct-detection (IM/DD) systems. The theoretical framework and preliminary simulations have been presented in Part I of this work. Here, the performance is compared between the GS FIR filter and the iterative GS algorithm in 56-Gb/s on-off keying (OOK) transmissions over 80-km single mode fiber (SMF) with a post feed-forward equalizer (FFE) for combating residual inter-symbol interference. Furthermore, the influence of the pulse shape (raised cosine or rectangular) and modulation format (return-to-zero (RZ) or non-return-to-zero (NRZ)) on the measured bit error ratio (BER) is investigated, while changing the number of FIR taps, post-FFE taps, and post-FFE samples-per-symbol. It is shown that within the range of the target digital extinction ratios (DERs) for which the original iterative GS algorithm offers benefit, both analytical and numerical methods for calculating the optimum FIR taps, outlined in Part I, produce similar BER performance as predicted. Hence, the former method is extended, here, through a non-recursive frequency response formula, which offers insight into the action of the GS filter with different pulse shaping and enables the derivation of the explicit GS impulse response. It is also shown that rectangular RZ exploits the full benefit of GS filtering through a uniform spectrum, achieving BER < 3.8×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup> , with a 641-tap <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> /2-spaced pre-EDC FIR filter and a 3-tap adaptive <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> -spaced post-FFE.

Probe pulse design in Brillouin optical time‐domain reflectometry

Brillouin optical time-domain reflectometry (BOTDR) is a branch of distributed fibre-optic sensors, and it can measure the strain and the temperature information, localised by the return time of the probe pulse, along the fibre based on the spontaneous Brillouin scattering process. Parameters of the BOTDR system, including the spatial resolution, the signal-to-noise ratio, the measurement speed, and the sensing range, have a mutually restrictive relationship. In order to improve the performance of the BOTDR system, researchers have focussed on improving the design of the probe pulse, for example, transforming the shape, the sequence, and the spectral properties of the pulse. This study summarises the recent progress in the design of detection pulses in BOTDR systems, and comprehensively demonstrates the improvement effects of various pulse modulation formats on the system performance.

Special issue “optical communications, sensing, and laser applications”

Special issue “optical communications, sensing, and laser applications”

A Novel Approach to Photonic Generation and Modulation of Ultra-Wideband Pulses

AbstractA novel approach to photonic generation of ultra-wideband (UWB) signals is proposed in this paper. The proposed signal generator is capable of generating UWB doublet pulses with flexible reconfigurability, and many different pulse modulation formats, including the commonly used pulse-position modulation (PPM) and bi-phase modulation (BPM) can be realized. Moreover, the photonic UWB pulse generator is capable of generating UWB signals with a tunable spectral notch-band, which is desirable to realize the interference avoidance between UWB and other narrow band systems, such as Wi-Fi. A mathematical model describing the proposed system is developed and the generation of UWB signals with different modulation formats is demonstrated via computer simulations.

Integrable high order UWB pulse photonic generator based on cross phase modulation in a SOA-MZI

We propose and experimentally demonstrate a potentially integrable optical scheme to generate high order UWB pulses. The technique is based on exploiting the cross phase modulation generated in an InGaAsP Mach-Zehnder interferometer containing integrated semiconductor optical amplifiers, and is also adaptable to different pulse modulation formats through an optical processing unit which allows to control of the amplitude, polarity and time delay of the generated taps.

Error ratio model for synchronised‐OOK IR‐UWB receivers in AWGN channels

Synchronised On/Off keying (S-OOK) is a pulse modulation format where timing synchronisation pulses are embedded with each bit in the data stream resulting in a 1.5 pulse/bit format, but allowing for synchronisation and demodulation with a single block. In this reported work, the error ratio for S-OOK symbols received in a non-fading AWGN channel is analysed and a mathematical model aware of the physical parameters of a CMOS integrated prototype is introduced. Using this model, the S-OOK RX sensitivity is shown to be close to that of a high-threshold OOK RX for similar error-ratios but assuming perfect synchronisation.

Analysis of pulse modulation format in coded BOTDA sensors

A theoretical and experimental analysis of the impact of pulse modulation format on Brillouin optical time-domain analysis (BOTDA) sensors using pulse coding techniques has been carried out. Pulse coding with conventional non-return-to-zero (NRZ) modulation format is shown to induce significant distortions in the measured Brillouin gain spectrum (BGS), especially in proximity of abrupt changes in the fiber gain spectra. Such an effect, as confirmed by the theoretical analysis, is due to acoustic wave pre-excitation and non-uniform gain which depends on the bit patterns defined by the different codewords. A successful use of pulse coding techniques then requires to suitably optimize the employed modulation format in order to avoid spurious oscillations causing severe penalties in the attained accuracy. Coding technique with return-to-zero (RZ) modulation format is analyzed under different duty-cycle conditions for a 25 km-long sensing scheme, showing that low duty-cycle values are able to effectively suppress the induced distortions in the BGS and allow for spatially-accurate, high-resolution strain and temperature measurements being able to fully exploit the provided coding gain (approximately 7.2 dB along 25 km distance) with unaltered spatial resolution (1 meter). Although Simplex coding is used in our analysis, the validity of the results is general and can be directly applied to any intensity-modulation coding scheme.

Optical UWB pulse generator using an N tap microwave photonic filter and phase inversion adaptable to different pulse modulation formats

We propose theoretically and demonstrate experimentally an optical architecture for flexible Ultra-Wideband pulse generation. It is based on an N-tap reconfigurable microwave photonic filter fed by a laser array by using phase inversion in a Mach-Zehnder modulator. Since a large number of positive and negative coefficients can be easily implemented, UWB pulses fitted to the FCC mask requirements can be generated. As an example, a four tap pulse generator is experimentally demonstrated which complies with the FCC regulation. The proposed pulse generator allows different pulse modulation formats since the amplitude, polarity and time delay of generated pulse is controlled.

Analysis of cross-phase modulation in WDM systems

A general theory on cross-phase modulation (XPM) intensity fluctuation has been derived. It is adapted to all kinds of modulation formats (continue wave, not-return-zero and return-zero pulse waves). In single-mode fibres (SMFs) and nonzero dispersion-shifted fibres (NZDSFs), this analytical solution for XPM is validated for the not-return-zero (NRZ) pulse modulation format. The relationship between the XPM intensity fluctuation and dispersion is discussed for different signal powers, channel numbers, wavelength separations, fibre lengths and nonlinear coefficients. The conclusions drawn can be used to explain some contradictory experimental results.

Impact of MPI and modulation format on transmission systems employing distributed Raman amplification

Measurements are performed of the impact of multipath interference (MPI) on a 40 Gbit/s CS-RZ system due to double Rayleigh scatter (DRS) noise. A new numerical simulation method for predicting the impact on Q of DRS noise in distributed Raman amplified systems, taking into account the pulse shape and modulation format, is also described. A simple analytical expression is presented which gives good agreement with the numerical simulation and includes modulation format and filtering. RZ formats are shown to be 1.5 to 2 dB more immune to MPI than NRZ formats.

Derivation of analytical expressions for the bit-error probability in lightwave systems with optical amplifiers

A description is given of a relatively simple derivation of the bit-error probability for a lightwave communications system using an amplitude-shift-keying (ASK) pulse modulation format and employing optical amplifiers such that amplified spontaneous emission noise dominates all other noise sources. Mathematically, this noise is represented as a Fourier series expansion with Fourier coefficients that are assumed to be independent Gaussian random variables. The bit-error probability is given in a closed analytical form that is derived by the approximate evaluation of several integrals appearing in the analysis. The author uses the theory to derive the Gaussian approximation and finds that it overestimates the bit-error rate by one to two orders of magnitude.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Average power constraints in AlGaAs semiconductor lasers under pulse-position-modulation conditions

In some optical communications systems there are advantages to using low duty-cycle pulse modulation formats such as pulse-position-modulation. However, because of intrinsic limitations of AlGaAs semiconductor lasers, the average power that they can deliver in a pulsed mode of operation is lower than in a CW mode. The magnitude of this problem and its implications are analyzed in this letter, and one possible solution is mentioned.

A Stripline-Implemented Baseband Signal Processor for Data and Clock Regeneration of PQM

A baseband signal processor for 1 Gbit/s pulse quaternary modulation (PQM), a pulse modulation format, has been developed and tested. A coupled stripline network is implemented for signal formatting, along with a frequency/phase tracking pulse demultiplexer which reduces predetection bandwidth and interpulse interference. Measured BER performance is presented along with computer simulation predictions.