Optical multicarrier sources for spectrally efficient optical networks

During the last 30 years the capacity of commercial optical systems exceeded the network traffic requirements, mainly due to the extraordinary scalability of wavelength division multiplexing technology that has been successfully used to expand capacity in optical systems and meet increasing bandwidth requirements since the early 1990’s. Nevertheless, the rapid growth of network traffic inverted this situation and current trends show faster growing network traffic than system capacity. To enable further and faster growth of optical communication network capacity, several breakthroughs occurred during the last decade. First, optical coherent communications, which were the subject of intensive research in the 1980’s, were revived. This triggered the employment of advanced modulation formats. Afterwards, with the introduction of orthogonal frequency division multiplexing (OFDM) and Nyquist WDM modulation techniques in optical communication systems, very efficient utilisation of the available spectral bandwidth was enabled. In such systems the spectral guard bands between neighbouring channels are minimised, at the expense of stricter requirements on the performance of optical sources, especially the frequency (or wavelength) stability. Attractive solutions to address the frequency stability issues are optical multicarrier sources which simultaneously generate multiple phase correlated optical carriers that ensure that the frequency difference between the carriers is fixed. In this thesis, a number of optical multicarrier sources are presented and analysed, with special focus being on semiconductor mode-locked lasers and gain-switched comb sources. High capacity and spectrally efficient optical systems for short and medium reach applications (from 3 km up to 300 km), based on optical frequency combs as optical sources, advanced modulation formats (m-QAM) and modulation techniques (OFDM and Nyquist WDM) have been proposed and presented. Also, certain optoelectronic devices (i.e. semiconductor optical amplifier) and techniques (feed-forward heterodyne linewidth reduction scheme) have been utilised to enable the desired system performance

Algorithms and Subsystems for Next Generation Optical Networks

This thesis investigates algorithms and subsystems for digital coherent optical networks to alleviate system requirements and enable spectrally efficient systems. Spectral shaping of individual channel is investigated to mitigate backreflections in bi-directional Passive Optical Network (PON) enabling more than 1000 users operating at 10 Gbit/s. It is then shown that temporal delay skews, caused by misalignment in the coherent receiver, induce a large penalty for Nyquist filtered signals. An adaptive 4×4 equaliser is developed to compensate the imperfections dynamically. This is subsequently demonstrated experimentally with Polarisation Division Multiplexed (PDM) Quadrature Phase Shift Keying (QPSK) and 16-level Quadrature Amplitude Modulation (QAM). Furthermore, a modified blind equaliser is designed to adaptively compensate for unknown amount of Chromatic Dispersion (CD). The equaliser is demonstrated experimentally using 10.7 GBd PDM-QPSK transmission over 5,200 km. To simplify the computational complexity of the equalisers a multiplier free update scheme is explored in simulations. Optical frequency combs are investigated as phase and frequency synchronised sub- carrier sources for Dense Wavelength Division Multiplexing (DWDM) systems. The effect of phase synchronisation between sub-channels of a superchannel is examined in simulations without showing performance deviation when no additional optical or digital processing is applied. Afterwards, the transmission performance of two generation techniques implementing 400 Gbit/s superchannels, utilising PDM-16QAM, is evaluated. Although, the average performance of the two combs is identical subchannel fluctuations are observed due to uneven spectral profile. Carrier Phase Estimation (CPE) is explored for both single channel and superchannels systems. An equaliser interleaved with CPE, is explored for PDM-64QAM with minor improvement. Alternatively, Digital Coherence Enhancement (DCE) allowed the detection of 6 GBd PDM-64QAM with a 1.4 MHz linewidth laser, an order of magnitude improvement in linewidth tolerance. Finally, a joint CPE across a comb superchannel is demonstrated with a factor of 5 tolerance improvement

Terabit-Rate Transmission Using Optical Frequency Comb Sources

Energy-efficient Tbit/s optical interconnects are key elements for future communication systems. Three novel optical frequency comb sources are investigated, which have the potential of being integrated in chip-scale Tbit/s transmitters. Such frequency combs provide a large number of carriers. The equidistance of the comb lines helps to minimize spectral guard bands. For each type of comb source, coherent data transmission experiments show the potential for Tbit/s data transmission rates

Digital Compensation of Transmission Impairments in Multi-Subcarrier Fiber Optic Transmission Systems

Time and again, fiber optic medium has proved to be the best means for transporting global data traffic which is following an exponential growth trajectory. Rapid development of high bandwidth applications since the past decade based on cloud, virtual reality, 5G and big data to name a few have resulted in a sudden surge of research activities across the globe to maximize effective utilization of available fiber bandwidth which until then was supporting low speed (< 10Gbps) services. To this end, higher order modulation formats together with multicarrier super channel based fiber optic transmission systems have proved to enhance spectral efficiency and achieve multi tera-bit per second bit rates. However, spectrally efficient systems are extremely sensitive to transmission impairments stemming from both optical devices and fiber itself. Therefore, such systems mandate the use of robust digital signal processing (DSP) to compensate and/or mitigate the undesired artifacts. The central theme of this research is to propose and validate few efficient DSP techniques to compensate specific impairments as delineated in the next three paragraphs. For short reach data center and passive optical network related applications which adopt direct detection, a single optical amplifier is generally used to meet the power budget requirements in order to achieve the desired receiver sensitivity or bit error ratio (BER). Semiconductor Optical Amplifier (SOA) with its small form factor is a low-cost power booster that can be designed to operate in any desired wavelength and more importantly can be integrated with other electro-optic components. However, saturated SOAs exhibit nonlinear amplification that introduce distortions on the amplified signal. Alongside SOA, the photodiode also introduces nonlinear mixing among the signal subcarriers in the form of Signal-Signal Beat Interference (SSBI). In this research, we study the impact of SOA nonlinearity on the effectiveness of SSBI compensation in a direct detection OFDM based transmission system. We experimentally demonstrate a digital compensation technique to undo the SOA nonlinearity effect by digitally backpropagating the received signal through a virtual SOA with inverse gain characteristics, thereby effectively eliminating the SSBI. With respect to transmission sources, laser technology has made some significant strides especially in the domain of multiwavelength sources such as quantum dot passive mode-locked laser (QD-PMLL) based optical frequency combs. In the present research work, we characterize the phase dynamics of comb lines from a QD-PMLL based on a novel multiheterodyne coherent detection technique. The inherently broad linewidth of comb lines which is on the order of tens of MHz make it difficult for conventional digital phase noise compensation algorithms to track the large phase noise especially for low baud rate subcarriers using higher cardinality modulation formats. In the context of multi-subcarrier, Nyquist pulse shaped, superchannel transmission system with coherent detection, we demonstrate through measurements and numerical simulations an efficient phase noise compensation technique called “Digital Mixing” that operates using a shared pilot tone exploiting mutual phase coherence among the comb lines. For QPSK and 16 QAM modulation formats, digital mixing provided significant improvement in BER performance in comparison to conventional phase tracking algorithms. Coherent solutions for regional and long haul systems make use of in-line optical amplifiers to compensate fiber loss. Ideally, distributed amplification based on stimulated Raman effect offers enhanced optical signal to noise ratios (OSNR) compared to lumped amplification using erbium doped fiber amplifiers and semiconductor optical amplifiers. However, this benefit of enhanced OSNRs in distributed Raman amplification is offset by the transfer of intensity noise of pump laser on to both signal’s phase and intensity, resulting in performance degradation. In this work, we propose and experimentally validate a practical pilot aided relative phase noise compensation technique for forward pumped distributed Raman amplified, digital subcarrier multiplexed coherent transmission systems

Chip-scale optical frequency comb sources for terabit communications

The number of devices connected to the internet and the required data transmission speeds are increasing exponentially. To keep up with this trend, data center interconnects should scale up to provide multi-Tbit/s connectivity. With typical distances from a few kilometers to 100 km, these links require the use of a high number of WDM channels. The associated transceivers should have low cost and footprint. The scalability of the number of channels, however, is still limited by the lack of adequate optical sources. In this book, we investigate novel chip-scale frequency comb generators as multi-wavelength light sources in WDM links. With a holistic model, we estimate the performance of comb-based WDM links, and we compare the transmission performance of different comb generator types, namely a quantum-dash mode-locked laser diode and a microresonator-based Kerr comb generator. We characterize their potential for massively-parallel WDM transmission with various transmission experiments. Combined with photonic integrated circuits, these comb sources offer a path towards highly scalable, compact, and energy-efficient Tbit/s transceivers

Subcarrier Multiplexing Based Transponder Design

This thesis presents the design and demonstration of high-speed transponders using analogue implemented subcarrier multiplexing (SCM) technique to simplify digital signal processing (DSP) for different applications. A 144-Gb/s filter bank multicarrier (FBMC) transceiver is numerically demonstrated for 2-km standard single mode fibre (SSMF) transmission. Without nonlinear or chromatic dispersion (CD) compensation nor channel equalization, the FBMC system outperforms the orthogonal frequency division multiplexing (OFDM) counterpart, and the transmission penalty for the 8-subcarrier FBMC system is 2.4 dB. For amplifier-free 80-km transmission, a 134-Gb/s coherent transceiver utilizing heterodyne detection and doubly differential (DD) quadrature phase shift keying (QPSK) is numerically demonstrated. Without CD compensation nor carrier recovery, transmission penalty and performance degradation for frequency offsets within ±2 GHz is negligible. To further improve interface rate, a 200-Gb/s DD QPSK transceiver using hybrid-assisted tandem single sideband (TSSB) modulation and digital coherent detection is numerically verified. However, guard bands and QPSK used in both transponders result in low spectral density, and conventional DD decoding degrades receiver sensitivity by 7 dB. To overcome these problems, a 209-Gb/s coherent transponder utilizing DD two amplitude/eight-phase shift keying (2ASK-8PSK) and 11-tap multi-symbol DD decoding is experimentally demonstrated, with an implementation penalty of 5.9 dB and a performance penalty of 1 dB for 100-km transmission. For long-haul application, a 62-GBaud SCM 16-ary quadrature amplitude modulation (16QAM) transceiver employing a single in-phase quadrature (IQ) mixer, simple transmitter-side DSP, and sub-band detection is demonstrated, giving spectral efficiency of ~2.7 b/s/Hz/polarization and OSNR penalty of 6.6 dB. By resorting to hybrid-assisted TSSB modulation, the aggregate symbol rate of the SCM transmitter is improved to 86 GBaud. With sub-band coherent detection and a 31-tap multi-input multi-output (MIMO) equalizer, an implementation penalty of 2 dB and spectral efficiency of ~3.6 b/s/Hz/polarization are achieved

Chip-scale optical frequency comb sources for terabit communications

To keep up with the ever-increasing data transmission speed needs, data center interconnects are scaling up to provide multi-Tbit/s connectivity. These links require a high number of WDM channels, while the associated transceivers should be compact and energy efficient. Scaling the number of channels, however, is still limited by the lack of adequate optical sources. In this book, we investigate novel chip-scale frequency comb generators as multi-wavelength light sources for Tbit/s WDM links

Chip-scale optical frequency comb sources for terabit communications

To keep up with the ever-increasing data transmission speed needs, data center interconnects are scaling up to provide multi-Tbit/s connectivity. These links require a high number of WDM channels, while the associated transceivers should be compact and energy efficient. Scaling the number of channels, however, is still limited by the lack of adequate optical sources. In this book, we investigate novel chip-scale frequency comb generators as multi-wavelength light sources for Tbit/s WDM links

Single-Laser Multi-Terabit/s Systems

Optical communication systems carry the bulk of all data traffic worldwide. This book introduces multi-Terabit/s transmission systems and three key technologies for next generation networks. A software-defined multi-format transmitter, an optical comb source and an optical processing scheme for the fast Fourier transform for Tbit/s signals. Three world records demonstrate the potential: The first single laser 10 Tbit/s and 26 Tbit/s OFDM and the first 32.5 Tbit/s Nyquist WDM experiments

Single-Laser Multi-Terabit/s Systems

Optical communication systems carry the bulk of all data traffic worldwide. This book introduces multi-Terabit/s transmission systems and three key technologies for next generation networks. A software-defined multi-format transmitter, an optical comb source and an optical processing scheme for the fast Fourier transform for Tbit/s signals. Three world records demonstrate the potential: The first single laser 10 Tbit/s and 26 Tbit/s OFDM and the first 32.5 Tbit/s Nyquist WDM experiments