Simultaneous clock recovery and data regeneration using a nonlinear optical loop mirror as an all-optical mixer

High-speed optical clock recovery, demultiplexing and data regeneration will be integral parts of any future photonic network based on high bit-rate OTDM. Much research has been conducted on devices that perform these functions, however to date each process has been demonstrated independently. A very promising method of all-optical switching is that of a semiconductor optical amplifier-based nonlinear optical loop mirror (SOA-NOLM). This has various advantages compared with the standard fiber NOLM, most notably low switching power, compact size and stability. We use the SOA-NOLM as an all-optical mixer in a classical phase-locked loop arrangement to achieve optical clock recovery, while at the same time achieving data regeneration in a single compact device

Simultaneous clock recovery and data regeneration using a nonlinear optical loop mirror as an all-optical mixer

High-speed optical clock recovery, demultiplexing and data regeneration will be integral parts of any future photonic network based on high bit-rate OTDM. Much research has been conducted on devices that perform these functions, however to date each process has been demonstrated independently. A very promising method of all-optical switching is that of a semiconductor optical amplifier-based nonlinear optical loop mirror (SOA-NOLM). This has various advantages compared with the standard fiber NOLM, most notably low switching power, compact size and stability. We use the SOA-NOLM as an all-optical mixer in a classical phase-locked loop arrangement to achieve optical clock recovery, while at the same time achieving data regeneration in a single compact device

Simultaneous demultiplexing, data regeneration, and clock recovery with a single semiconductor optical amplifier-based nonlinear-optical loop mirror

We demonstrate simultaneous demultiplexing, data regeneration and clock recovery at 10Gbits/s, using a single semiconductor optical amplifier–based nonlinear-optical loop mirror in a phase-locked loop configuration.

Simultaneous demultiplexing, data regeneration, and clock recovery with a single semiconductor optical amplifier-based nonlinear-optical loop mirror

We demonstrate simultaneous demultiplexing, data regeneration and clock recovery at 10Gbits/s, using a single semiconductor optical amplifier–based nonlinear-optical loop mirror in a phase-locked loop configuration.

Drop and insert multiplexing with simultaneous clock recovery using an electroabsorption modulator

A three-node optical time-division multiplexing (OTDM) network is demonstrated that utilizes electroabsorption (EA) modulators as the core elements. Each node is self contained and performs its own clock recovery and synchronization. “Drop and insert” functionality is demonstrated for the first time with an EA modulator by completely removing a 10-Gb/s channel from a 40-Gb/s OTDM data stream. A different 10-Gb/s channel was subsequently inserted into the vacant time slot. Clock recovery is achieved by using an EA modulator in a novel bidirectional configuration. Bit-error-rate (BER) measurements are presented for each of the 10-Gb/s OTDM channels.

Drop and insert multiplexing with simultaneous clock recovery using an electroabsorption modulator

A three-node optical time-division multiplexing (OTDM) network is demonstrated that utilizes electroabsorption (EA) modulators as the core elements. Each node is self contained and performs its own clock recovery and synchronization. "Drop and insert" functionality is demonstrated for the first time with an EA modulator by completely removing a 10-Gb/s channel from a 40-Gb/s OTDM data stream. A different 10-Gb/s channel was subsequently inserted into the vacant time slot. Clock recovery is achieved by using an EA modulator in a novel bidirectional configuration. Bit-error-rate (BER) measurements are presented for each of the 10-Gb/s OTDM channels.

Simultaneous demultiplexing and clock recovery using a single electroabsorption modulator in a novel bi-directional configuration

A single electroabsorption modulator was used to demultiplex a 10 Gbit/s channel from a 40 Gbit/s OTDM data stream, whilst simultaneously recovering the 10 GHz electrical clock. This was achieved using a new bi-directional operation of the EA modulator, combined with a simple phase-locked loop feedback circuit. Excellent system performance was achieved, indicating that operation up to and beyond 100 Gbit/s is possible using current technology.

Simultaneous demultiplexing and clock recovery using a single electroabsorption modulator in a novel bi-directional configuration

A single electroabsorption modulator was used to demultiplex a 10 Gbit/s channel from a 40 Gbit/s OTDM data stream, whilst simultaneously recovering the 10 GHz electrical clock. This was achieved using a new bi-directional operation of the EA modulator, combined with a simple phase-locked loop feedback circuit. Excellent system performance was achieved, indicating that operation up to and beyond 100 Gbit/s is possible using current technology.

All optical networking beyond 10 Gbits/s OTDM networks based on electro-optic modulators and fibre ring lasers

This thesis examines options for high capacity all optical networks. Specifically optical time division multiplexed (OTDM) networks based on electro-optic modulators are investigated experimentally, whilst comparisons with alternative approaches are carried out. It is intended that the thesis will form the basis of comparison between optical time division multiplexed networks and the more mature approach of wavelength division multiplexed networks. Following an introduction to optical networking concepts, the required component technologies are discussed. In particular various optical pulse sources are described with the demanding restrictions of optical multiplexing in mind. This is followed by a discussion of the construction of multiplexers and demultiplexers, including favoured techniques for high speed clock recovery. Theoretical treatments of the performance of Mach Zehnder and electroabsorption modulators support the design criteria that are established for the construction of simple optical time division multiplexed systems. Having established appropriate end terminals for an optical network, the thesis examines transmission issues associated with high speed RZ data signals. Propagation of RZ signals over both installed (standard fibre) and newly commissioned fibre routes are considered in turn. In the case of standard fibre systems, the use of dispersion compensation is summarised, and the application of mid span spectral inversion experimentally investigated. For green field sites, soliton like propagation of high speed data signals is demonstrated. In this case the particular restrictions of high speed soliton systems are discussed and experimentally investigated, namely the increasing impact of timing jitter and the downward pressure on repeater spacings due to the constraint of the average soliton model. These issues are each addressed through investigations of active soliton control for OTDM systems and through investigations of novel fibre types respectively. Finally the particularly remarkable networking potential of optical time division multiplexed systems is established, and infinite node cascadability using soliton control is demonstrated. A final comparison of the various technologies for optical multiplexing is presented in the conclusions, where the relative merits of the technologies for optical networking emerges as the key differentiator between technologies.

OTDM network processing using all-optical and electro-optical devices

The current optical communications network consists of point-to-point optical transmission paths interconnected with relatively low-speed electronic switching and routing devices. As the demand for capacity increases, then higher speed electronic devices will become necessary. It is however hard to realise electronic chip-sets above 10 Gbit/s, and therefore to increase the achievable performance of the network, electro-optic and all-optic switching and routing architectures are being investigated. This thesis aims to provide a detailed experimental analysis of high-speed optical processing within an optical time division multiplexed (OTDM) network node. This includes the functions of demultiplexing, 'drop and insert' multiplexing, data regeneration, and clock recovery. It examines the possibilities of combining these tasks using a single device. Two optical switching technologies are explored. The first is an all-optical device known as 'semiconductor optical amplifier-based nonlinear optical loop mirror' (SOA-NOLM). Switching is achieved by using an intense 'control' pulse to induce a phase shift in a low-intensity signal propagating through an interferometer. Simultaneous demultiplexing, data regeneration and clock recovery are demonstrated for the first time using a single SOA-NOLM. The second device is an electroabsorption (EA) modulator, which until this thesis had been used in a uni-directional configuration to achieve picosecond pulse generation, data encoding, demultiplexing, and 'drop and insert' multiplexing. This thesis presents results on the use of an EA modulator in a novel bi-directional configuration. Two independent channels are demultiplexed from a high-speed OTDM data stream using a single device. Simultaneous demultiplexing with stable, ultra-low jitter clock recovery is demonstrated, and then used in a self-contained 40 Gbit/s 'drop and insert' node. Finally, a 10 GHz source is analysed that exploits the EA modulator bi-directionality to increase the pulse extinction ratio to a level where it could be used in an 80 Gbit/s OTDM network.

40 Gbit/s asynchronous digital optical regenerator

We present the first experimental demonstration of an asynchronous digital optical regenerator at 42.67 Gbit/s. The system effectively retimes incoming asynchronous data bursts to a local clock without burst mode clock recovery and converts the signal to a desired wavelength and duty cycle.

Optical injection-locking-based pump recovery for phase-sensitively amplified links

An injection-locking-based pump recovery system for phase-sensitively amplified links is proposed and studied experimentally. Measurements with 10 Gbaud DQPSK signals show penalty-free recovery of 0.8 GHz FWHM bandwidth pump with 63 dB overall amplification. © 2012 OSA.

Semiconductor optical amplifier-based nonlinear optical-loop mirror with feedback

The behavior of a semiconductor optical amplifier (SOA)-based nonlinear loop mirror with feedback has been investigated as a potential device for all-optical signal processing. In the feedback device, input signal pulses (ones) are injected into the loop, and amplified reflected pulses are fed back into the loop as switching pulses. The feedback device has two stable modes of operation - block mode, where alternating blocks of ones and zeros are observed, and spontaneous clock division mode, where halving of the input repetition rate is achieved. Improved models of the feedback device have been developed to study its performance in different operating conditions. The feedback device could be optimized to give a choice of either of the two stable modes by shifting the arrival time of the switching pulses at the SOA. Theoretically, it was found possible to operate the device at only tens of fJ switching pulse energies if the SOA is biased to produce very high gain in the presence of internal loss. The clock division regime arises from the combination of incomplete SOA gain recovery and memory of the startup sequence that is provided by the feedback. Clock division requires a sufficiently high differential phase shift per unit differential gain, which is related to the SOA linewidth enhancement factor.

Recovery of multiple amplitude levels from a DQPSK NRZ signal using a single Mach-Zehnder Interferometer

In this research the recovery of a DQPSK signal will be demonstrated using a single Mach-Zehnder Interferometer (MZI). By changing the phase delay in one of the arms it will be shown that different delays will produce different output levels. It will also be shown that with a certain level of phase shift the DQPSK signal can be converted into four different equally spaced optical power levels. With each decoded level representing one of the four possible bit permutations. By using this additional phase shift in one of the arms the number of MZIs required for decoding can be reduced from two to one.

All-optical processing using cross-phase modulation

The following thesis presents results obtained from both numerical simulation and laboratory experimentation (both of which were carried out by the author). When data is propagated along an optical transmission line some timing irregularities can occur such as timing jitter and phase wander. Traditionally these timing problems would have been corrected by converting the optical signal into the electrical domain and then compensating for the timing irregularity before converting the signal back into the optical domain. However, this thesis posses a potential solution to the problem by remaining completely in the optical domain, eliminating the need for electronics. This is desirable as not only does optical processing reduce the latency effect that their electronic counterpart have, it also holds the possibility of an increase in overall speed. A scheme was proposed which utilises the principle of wavelength conversion to dynamically convert timing irregularities (timing jitter and phase wander) into a change in wavelength (this occurs on a bit-by-bit level and so timing jitter and phase wander can be compensated for simultaneously). This was achieved by optically sampling a linearly chirped, locally generated clock source (the sampling function was achieved using a nonlinear optical loop mirror). The data, now with each bit or code word having a unique wavelength, is then propagated through a dispersion compensation module. The dispersion compensation effectively re-aligns the data in time and so thus, the timing irregularities are removed. The principle of operation was tested using computer simulation before being re-tested in a laboratory environment. A second stage was added to the device to create 3R regeneration. The second stage is used to simply convert the timing suppressed data back into a single wavelength. By controlling the relative timing displacement between stage one and stage two, the wavelength that is finally produced can be controlled.