50-GHz Repetition Gain Switching Using A Cavity-enhanced DFB Laser Assisted By Optical Injection Locking

We demonstrate pulse generation at a repetition rate of 50 GHz by gain switching an injection-locked distributed feedback (DFB) laser with large modulation bandwidth (BW). The small-signal BW of the DFB laser was extended from an intrinsic BW of 30 GHz to 52 GHz due to the joint effects of photon-photon resonance (PPR) and detuned-loading, which exploits the frequency-dependent cavity loss to enhance the effective differential gain and thereby the relaxation oscillation frequency. The results show that PPR and detuned-loading effects can also be utilized to improve large signal gain-switching, despite the dynamic changes of the detuned loading condition due to the large signal chirp. By modulating the laser with 50-GHz RF signals, we obtained 50-GHz repetition rate pulses from the gain-switched laser, confirming that the BW enhancement effects are still valid for large-signal modulation. After that, we optically injection lock the gain-switched laser with strong external seeding light, which suppresses the chirp and creates frequency tones with 50 GHz spacing. The optical injection locking (OIL) further improves the large-signal BW, resulting in narrower pulse width. The spectrum of the gain-switched OIL directly modulated laser (OIL-DML) showed three sideband peaks above 70% from the peak (full width half maximum of 130 GHz), and a pulse width of 9.5 ps (6.2 ps after deconvolution) was obtained. The results indicate that the pulse could be compressed to 2.4 ps, assuming linear chirp

Advanced Digital Signal Processing Techniques for High-Speed Optical Links

L'abstract è presente nell'allegato / the abstract is in the attachmen

Developing coherent optical wavelength conversion systems for reconfigurable photonic networks

In future optical networks that employ wavelength division multiplexing (WDM), the use of optical switching technologies on a burst or packet level, combined with advanced modulation formats would achieve greater spectral efficiency and utilize the existing bandwidth more efficiently. All-optical wavelength converters are expected to be one of the key components in these broadband networks. They can be used at the network nodes to avoid contention and to dynamically allocate wavelengths to ensure optimum use of fiber bandwidth. In this work, a reconfigurable wavelength converter comprising of a Semiconductor Optical Amplifier (SOA) as the nonlinear element and a fast-switching sampled grating distributed Bragg reflector (SG-DBR) tunable laser as one of the pumps is developed. The wavelength conversion of 12.5-Gbaud quadrature phase shift keying (QPSK) and Pol-Mul QPSK signals with switching time of tens of nanoseconds is experimentally achieved. Although the tunable DBR lasers can achieve ns tuning time, they present relatively large phase noise. The phase noise transfer from the pump to the converted signal can have a deleterious effect on signal quality and cause a performance penalty with phase modulated signals. To overcome the phase noise transfer issue, a wavelength converter using tunable dual-correlated pumps provided by the combination of a single-section quantum dash passively mode-locked laser (QD-PMLL) and a programmable tunable optical filter is designed and the wavelength conversion of QPSK and 16-quadrature amplitude modulation (16-QAM) signals at 12.5 GBaud is experimentally investigated. Nonlinear distortion of the wavelength converted signal caused by gain saturation effects in the SOA can significantly degrade the signal quality and cause difficulties for the practical wavelength conversion of sig nal data with advanced modulation formats. In this work, the machine learning clustering based nonlinearity compensation method is proposed to improve the tolerance to nonlinear distortion in an SOA based wavelength conversion system with 16 QAM and 64 QAM signals

Real-Time DSP-Free 100Gbit/s/λ PAM-4 Fiber Access Link using EML and Direct Detection

A 100 Gbit/s/ λ PAM-4 fiber link with an optical budget of 30 dB and 20 km fiber reach is achieved in real time experiments. This is compliant with class A (20 dB) point to point (PtP) applications as mobile fronthaul for example, and with class N1 (29 dB) point to multipoint (PtMP) for residential market. We used an integrated externally modulated laser, an analog pre-equalizer, an optical booster amplifier and/or non-filtered preamplifier and direct detection without any digital signal processing (whether real-time or offline)

Next-generation High-Capacity Communications with High Flexibility, Efficiency, and Reliability

The objective of this dissertation is to address the flexibility, efficiency and reliability in high-capacity heterogeneous communication systems. We will experimentally investigate the shaping techniques, and further extend them to more diverse and complicated scenarios, which result in more flexible systems. The scenarios include 1) entropy allocation scheme under uneven frequency response for multi-carrier system, 2) fiber-free space optics link using unipolar pairwise distribution, and 3) flexible rate passive optical network with a wide range of received optical powers. Next, we perform efficiency analysis in inter-data center and long-haul communications. We will characterize the impact of the laser linewidth, jitter tones, and the flicker noise on coherent systems with different baud rates and fiber lengths through theoretical analysis, simulation, and experimental validation. The trade-off analysis indicates the importance of setting up frequency noise power spectral density masks to qualify the transceiver laser design. Besides efficiency analysis, we will also work on efficient system architecture and algorithm design. We investigate the combined impact of various hardware impairments using proposed simplified DSP schemes in beyond 800G self-homodyne coherent system. The proposed scheme is very promising for next-generation intra-data center applications. On the other hand, to improve the data efficiency of the nonlinearity correction algorithm in broadband communication systems, we leverage the semi-supervised method and Lasso. Experimental results validate that Lasso can reduce the required pilot symbol number by exploiting the sparsity of the tap coefficients. Semi-supervised method can further enhance the system performance without introducing additional overhead. Last but not least, regarding reliability, we propose and experimentally demonstrate an ultra-reliable integrated millimeter wave and free space optics analog radio over fiber system with algorithm design. The multiple-spectra operation shows superior performance in reliability and sensitivity compared to the conventional systems, even in extreme weather conditions and strong burst interference.Ph.D

Development of high capacity transmission systems for future optical access networks

The cost-sensitivity of NG-PON2 and future optical access networks, employing wavelength division multiplexing (WDM) technology, may preclude the use of conventional LiNbO3-based intensity and I/Q modulators, as they are currently too expensive for use in the access domain. Cost-effective directly modulated lasers (DMLs) and electro-absorption modulated lasers (EMLs) will need to be employed and, thus, are expected to be integral components in the realisation of tunable laser sources for future optical access networks. The limitations of DMLs and EMLs as transmitters merit thorough investigation to further understand how these devices can be adapted or optimised for use as tunable laser sources in future optical access networks. In this thesis, the transmission performance of a directly modulated DFB laser (DML) and an externally modulated DFB laser monolithically integrated with an EAM (EML), are investigated. The performance of both devices under 12.5 Gbit/s NRZ-OOK modulation are evaluated for transmission over standard single-mode fibre (SSMF) in an IM/DD test-bed, with a view to further understanding the limitations of DMLs and EMLs in 10 Gbit/s IM/DD systems. Particular attention is given to the frequency chirp of the devices and how the chirp affects the performances of the devices for transmission over SSMF up to 50 km in length. Numerical models, which were developed in MATLAB, are utilised to simulate the characteristics and transmission performances of both the DML and EML. The latter half of this thesis is focused on the development of a self-seeded Fabry-Pérot (SS-FP) laser. The SS-FP laser is optimised and characterised, and the transmission performance of the directly modulated SS-FP laser over SSMF is evaluated in an IM/DD test-bed. Two intensity modulation (IM) formats are assessed, 12.5 Gbit/s NRZ-OOK and 12.5 Gbaud/s (25 Gbit/s) multilevel PAM-4, both IM formats are compatible with 10G class optical components and legacy PON deployments. The SS-FP laser holds potential for photonic integration, justifying its consideration as a candidate tunable laser source for next generation PONs and future optical access networks