On-farm anaerobic digestion plants served by virtual biomethane pipelines may be used in areas with no gas grid infrastructure access to improve manure (slurry) management and to produce and deliver renewable gas. A modelling analysis of a simulated virtual biomethane pipeline is presented whereby gas is collected from 100 onfarm biogas plants fed with slurry and grass silage feedstocks and delivered to a market entry point (depot). In total, 7 scenarios are presented in the analysis. In scenarios 1-4, the biogas is upgraded on-site by the digester operator, the biomethane is collected by the haulage vehicle(s) and transported to a single depot. The impact of an additional depot on vehicle routing is investigated in scenarios 5 and 6. Scenario 7 investigates the use of a mobile-upgrading and compression unit, which would allow biogas plant operators to forego the installation and operation of biogas upgrading facilities. In scenario 3 (which considers the largest capacity biomethane haulage vehicles and a single delivery depot) all 100 anaerobic digestion plants are visited over the course of one week using 4 routes, with a total aggregated distance travelled of 205 km. Scenario 7 requires a total routing distance in excess of 613 km to visit all 100 biogas plants and requires long routing times and periods of downtime at each site due to the slow upgrading speed of the mobile unit. The weekly greenhouse gas emissions due to vehicle routing from each scenario are calculated. Assuming diesel-fuelled haulage vehicles, scenario 7 routing results in 431 kg CO2-eq week-1, a 167% increase in vehicle route emissions compared to scenario 3 (161 kg CO2-eq week-1). Additionally, whilst the mobile-upgrading unit may alleviate financial challenges, they carry significant logistical challenges, and their resultant higher routing emissions will impact the sustainability of the collected biomethane. Biomethane fuelled haulage vehicles may address this concern if the biomethane is sustainably sourced.

Purpose This paper provides insights into national practices used to schedule, collect and manage the transportation infrastructure of raw milk by Irish processors. Design/methodology/approach A survey was designed and distributed to 14 processors, collecting details regarding suppliers, seasonality, costs per litre, planning, processing sites and emissions related to milk collection. Findings Irish raw milk transportation costs €95 million per annum, with an average weighted cost of 1.1 cents per litre. Primary route clustering of suppliers is based on farm location. Typically, collections employ forty-eight-hour rotas. Just three of the processors reported transportation emissions data. A disjointed approach to the adoption of scheduling and transportation technology was revealed. Research limitations/implications Given the broad scope of the survey covering financial, operational and environmental aspects of milk collection, it was challenging to find a single representative such as a transport manager who could be tasked with responding to the entire survey. Future research may consider a more focused interview-based approach with the various stakeholders to provide a more in-depth analysis. Practical implications Processors can gain an improved understanding of diversified milk collection methods. The research supports policymakers in considering environmental issues related to milk transportation. Costs could be reduced if transportation was better managed collectively with benefits accruing to the industry, suppliers and wider rural community. Stakeholders will need to address aspects of responsibility concerning environmental issues going forward. Social implications In this paper the authors recognise the environmental cost of milk collection. By improving the transportation infrastructure, this will have a positive impact on society in general. Originality/value The paper highlights the unique challenges and extends present knowledge in relation to milk collection; thus, this paves the way for new approaches to raw milk transportation.

This paper presents a seasonal transportation of raw milk (SToRM) model. Irish raw milk collection is estimated to annually cost in excess of €80 million. Employing a capacitated vehicle routing problem (CVRP) methodology and incorporating geographic information system (GIS) data, SToRM simulates a two‐day collection schedule using the Irish road network. Differences of over 60% in total route distance required between January (trough) and May (peak) were observed. However, varied seasonal patterns did not dramatically affect overall transport needs. Results highlight the need for dynamic scheduling. SToRM provides practitioners access to a powerful model for use in milk collection scheduling.

Quantum dot lasers have proven to be extremely interesting in terms of nonlinear dynamics. One of the key benefits is their highly damped relaxation oscillations. In this work we take advantage of this stability in a master slave configuration. We find a novel, dual state excitable dynamic resulting in antiphase operation between two lasing states in the device. We interpret the behaviour as a non-Adler but still Type I excitability. Phase measurements bolster our interpretation.

Quantum dot lasers display many unique dynamic phenomena when optically injected. Bistability has been predicted in a region of high injection strength. Experimentally, we show that a square wave phenomenon, rather than a phase-locked bistability, is observed in this region. The squares can manifest as a periodic train but also as noise-driven Type II excitable events. We interpret the appearance of the square waves as a thermally induced breaking of the bistability. Indeed, we find experimentally that over the duration of a square, the relative detuning between the master and the slave evolves deterministically. A relatively simple, physically motivated, rate equation model is presented and displays excellent agreement with the experiment.

Summary form only given. Quantum dot (QD) lasers display the unique ability to lase from multiple energy states [1]. Depending on pump current a QD laser can lase from the ground state (GS), from the first excited state (ES), or simultaneously from both. Although a lot of work has been done investigating the neuromorphic dynamics for the GS state, very little has been done involving the ES. In [2] antiphase ES and GS bursting intensity oscillations were reported while in [3], a novel Type I dual state excitable dynamic was reported. Here we show that we can controllably produce GS dark pulses and an associated ES bright pulse using sufficiently large perturbations to deterministically trigger them. We also identify and show techniques that can be used to control the number of spikes in the bursts and the refractory time in the dual state excitability.

An experimental study into the modal dynamics of a short cavity, fast frequency-swept laser is presented. This commercially available external cavity swept source is designed for use in optical coherence tomography (OCT) applications and displays a number of dynamic lasing regimes during the course of the wavelength sweep. Interferometric full electric field reconstruction is employed, allowing for measurement of the laser operation in a time-resolved, single-shot manner. Recovery of both the phase and intensity of the laser output across the entire sweep enables direct visualization of the laser instantaneous optical spectrum. The electric field reconstruction technique reveals the presence of multi-mode dynamics, including coherent mode-locked pulses. During the main part of the imaging sweep, the laser is found to operate in a second harmonic sliding frequency mode-locking regime. Examination of the modal evolution of this coherent regime reveals evidence of previously unobserved frequency switching dynamics.

Excitable pulses are one of the necessary building blocks in achieving a functional neuromorphic photonic system. Many different semiconductor devices have been arranged in various configurations to produce excitable pulses. Of these, Quantum Dot (QD) lasers in a unidirectional optical injection set-up are great candidates for future applications. The optically injected QD laser is shown to be a truly unique set up producing a plethora of different non linear dynamics, many of which are potentially useful in neuromorphic applications. In particular, there are several different excitable regimes: Both Type I and Type II excitability can be produced. In fact, both types are produced with exactly the same master-slave configuration. By simply adjusting the control parameters only - injection strength and detuning - it is easy to navigate between these two types of excitability.

We study the dynamics of semiconductor lasers subject to strong delayed optical feedback. We find that the discrete nature of the external-cavity modes leads to a discrete set of relaxation oscillation frequencies observed at the onset of the first instability as the pump current is increased. This explains experimentally observed hops between the frequencies.

Injection locking hasmany applications in telecommunications systems, such as narrowing linewidths, increasing bandwidth and improving filtering. Beyond telecommunications, injection locking is widely used in remote sensing. This is of particular interest for applications in the 2 μm region, where gases such as carbon dioxide, water vapour andmethane have identifiable absorption features. In this paper, we demonstrate stable injection lockingwith slotted Fabry–Perot lasers in the 2 μmwavelength region. Injection lockingwas observed in both the optical domain and power spectrum;with key features recorded such as injection ‘pulling’, side-mode suppression and the characteristic quiet region in the electrical domain denoting single-frequency emission and stable locking. The effect of varying the injection ratio was investigated, with a decreased injection ratio corresponding to a reduction in the locking bandwidth. Finally, the lasers were shown to remain injection locked, with no thermal drift, for over 24 h, indicating their suitability for implementation in a real-world telecommunications system.

We explore both experimentally and numerically the dynamics of semiconductor lasers subject to delayed optical feedback and show that the external cavity repetition rate can be resonant with the relaxation oscillations leading to a discretisation of the relaxation oscillation frequency which evolves in a series of discrete steps, remaining almost constant along each step. Numerically, the steps are found to result from different Hopf bifurcation branches.

This paper presents an innovative desktop milk collection route simulation model. The simulation focuses on the development of an optimal routing plan for a designated fleet of articulated lorries (trucks) and purpose-built tankers used in the collection, transportation and varying seasonal delivery patterns of milk from farms to processors over a national road network vis-à-vis, the Irish road network. A brief background of the significant role that ever-evolving technology has had on the dairy industry is presented. Followed by an outline of the multi constituents that form the complex milk assembly process. We move on to describe a model which has been designed to robustly cater for the specific characteristics that uniquely occur within the Irish dairy industry. The research presented suggests the formulation of a paradigm to accommodate the dynamic nature of the Irish milk collection process. A modified adaptation of a more ubiquitous solution, that was devised to encompass a broad-spectrum approach tasked with the efficient solving of numerous variations of the well-known Vehicle Routing Problem (VRP) across multiple environments, is implemented. Employing the widely used heuristic technique known as the large neighbourhood search (LNS) algorithm, a new model has been conceived and tested that allows for the formation of scheduled collection and delivery routes that can efficiently resolve routing problems that are distinctive to the dairy industry and principally those of the Irish industry. With issues such as Brexit now facing the Irish dairy industry this paper aims to put forward an efficient model to support those involved at various levels within the milk transportation sector of the industry. Furthermore, the model has been designed with the intention that future research can use this base model as a cornerstone for more in-depth research in the ever-evolving area of milk assembly.

We study the dynamics of semiconductor lasers subject to strong delayed optical feedback. We find that the discrete nature of the external-cavity modes leads to a discrete set of relaxation oscillation frequencies observed at the onset of the first instability as the pump current is increased. This explains experimentally observed hops between the frequencies.

In this paper, we demonstrate optical injection locking in the 2 µm wavelength region between two discrete semiconductor lasers. We will highlight how such behaviour can be used as a key enabling technology in this new wavelength window, motivated by system improvements such as better filtering and linewidth reduction for advanced modulation formats.

Multiple time scales appear in many nonlinear dynamical systems. Semiconductor lasers, in particular, provide a fertile testing ground for multiple time scale dynamics. For solitary semiconductor lasers, the two fundamental time scales are the cavity repetition rate and the relaxation oscillation frequency which is a characteristic of the field-matter interaction in the cavity. Typically, these two time scales are of very different orders, and mutual resonances do not occur. Optical feedback endows the system with a third time scale: the external cavity repetition rate. This is typically much longer than the device cavity repetition rate and suggests the possibility of resonances with the relaxation oscillations. We show that for lasers with highly damped relaxation oscillations, such resonances can be obtained and lead to spontaneous mode-locking. Two different laser types--a quantum dot based device and a quantum well based device-are analysed experimentally yielding qualitatively identical dynamics. A rate equation model is also employed showing an excellent agreement with the experimental results.

A real-time study of the dynamic properties of a frequency swept vertical cavity surface emitting laser (VCSEL) is presented. Such tunable laser sources have previously been shown to provide long coherence lengths and improved performance in metrology and imaging applications. Single-shot interferometric reconstruction of both the phase and intensity of the swept source allows for experimental measurement of the full complex electric field over multiple sweep periods. Access to the electric field enables direct measurement of laser properties that can be related to the imaging performance of the laser when used in a swept source optical coherence tomography application. Both inter-sweep and intra-sweep characterization is possible, including determination of the instantaneous spectral shape, sweep rate, linewidth, coherence roll-off, and point spread function.

Neurons communicate by brief bursts of spikes separated by silent phases and information may be encoded into the burst duration or through the structure of the interspike intervals. Inspired by the importance of bursting activities in neuronal computation, we have investigated the bursting oscillations of an optically injected quantum dot laser. We find experimentally that the laser periodically switches between two distinct operating states with distinct optical frequencies exhibiting either fast oscillatory or nearly steady state evolutions (two-color bursting oscillations). The conditions for their emergence and their control are analyzed by systematic simulations of the laser rate equations. By projecting the bursting solution onto the bifurcation diagram of a fast subsystem, we show how a specific hysteresis phenomenon explains the transitions between active and silent phases. Since size-controlled bursts can contain more information content than single spikes our results open the way to new forms of neuron inspired optical communication.

Bursting outputs are important for neuronal communication [1] and arise via a coupling between two basic mechanisms: the first generating fast pulsations and the second a slow switching between quiescent and active phases. While well studied in mathematical biology, the phenomenon is rare in laser systems.

A time-resolved study is presented of the single-mode and mode-switching dynamics observed in swept source vertical cavity surfing emitting lasers and swept wavelength short external cavity lasers. A self-delayed interferometric technique is used to experimentally measure the phase and intensity of these frequency swept lasers, allowing direct examination of the modal dynamics. Visualisation of the instantaneous optical spectrum reveals mode-hop free single mode lasing in the case of the vertical cavity laser, with a tuning rate of 6.3 GHz/ns. More complex mode-switching behaviour occurs in the external cavity laser, with the mode-hopping dynamics found to be dominated by the deterministic movement of the spectral filter. Evidence of transient multi-mode operation and mode-pulling is also presented.

Optical injection into the ground state (GS) of a quantum dot laser nominally emitting from the excited state (ES) can induce a switch to GS emission. Antiphase oscillations are obtained consisting of short ES pulses with GS dropouts. These disappear when the master laser is sufficiently detuned or weak and CW emission from the (multimode) ES is regained. The pulses are accompanied by a notable expansion of the ES optical spectrum, which disappears abruptly when the system switches to the CW ES operation. The phenomenon arises due to the large difference in timescales between the ns scale pulses and the ps scale carrier distribution times of the quantum dots.