Digital imaging has traditionally been enabled by single-crystalline photodetectors. This approach typically either mandates the use of silicon as photon-to-electron converter or requires a hybrid-integrated solution. In contrast, solution-processed optoelectronic materials offer convenient integration of light-sensing materials atop an electronic readout circuit. Colloidal quantum dots offer particular advantages, combining solution-processing with the spectral tunability afforded by the quantum size effect. Here we review recent progress in solution-processed quantum dot photodetectors and their application in future imaging systems. We focus on progress towards high responsivity (photon-to-electron gains exceeding 1000) and sensitivity (normalized detectivity D* ~ 1013Jones) in the visible, the near infrared, and the short-wavelength infrared. We also highlight the achievement of solution-processed photoconductive photodetectors combining photoconductive gain and temporal responses faster than 30 ms, devices therefore compatible with video-frame-rate imaging. We conclude with a discussion of recent colloidal quantum dot photodiodes having megahertz bandwidth and detectivity of 1011Jones.

Physically flexible, solution-processed solar cells have the potential to harvest light conveniently, cheaply and efficiently. Absorption of the considerable power in the infrared region of the solar spectrum is necessary for efficient solution-processed solar cells. We use lead sulphide (PbS) colloidal nanoparticles (quantum dots) that are quantum size effect tunable, allowing tailoring of absorption onset between 900 and 2000 nm. A number of device architectures will be described within, leading to photovoltaic devices which exhibit external quantum efficiencies exceeding 1%.

In this paper, we introduce a new signalling architecture called Dual-header Optical Burst Switching (DOBS) for next generation burst-switching optical networks. DOBS decouples the resource reservation process from the service request process in core nodes and allows for delayed scheduling to be implemented. This relaxes the constraints on burst scheduling operations and allows the offset sizes of bursts to be precisely controlled in core nodes without the use of fiber delay line buffers. This allows for increased flexibility, control, and performance. To demonstrate the benefit of delayed scheduling and core. node offset control, we examine the performance of a DOBS system in which the offset size of every burst on a core link is set to a constant value. Using simulation and analysis, we show that the resulting constant-scheduling-offset (CSO) system realizes lower ingress delay, higher throughput, and better fairness than conventional single-header OBS systems, while simultaneously requiring only O(1) burst scheduling complexity. In a 16-channel system with full wavelength conversion and no fiber delay line buffers, the CSO DOBS system achieved a blocking probability 50% lower than that of a similar LAUC-VF JET OBS system. The CSO DOBS system also achieved perfect fairness, both with respect to burst length and with respect to the residual path length of bursts

In this paper we introduce a new signalling architecture called dual-header optical burst switching (DOBS) for next generation burst-switching optical networks. Using DOBS, the functional offset size of every burst on a given link can be set to the same size without the use of fiber delay line buffers. This allows DOBS to realize lower burst-scheduling complexity, lower ingress delay, higher throughput and better fairness than conventional single-header OBS systems. We present a new burst-scheduling algorithm called free channel queue scheduling that requires only O(1) time to execute and that achieves optimal performance in constant-offset DOBS systems. Using simulation, we find that the blocking probability of a 16-channel DOBS system is 50% lower than that of a similar LAUC-VF JET OBS system. We also show that DOBS achieves better fairness than JET OBS with respect to burst length and with respect to the residual path length of bursts.

In this paper, we present for the first time an analytical model that quantifies the mechanism by which offset size affects priority in multiclass optical-burst switching (OBS) systems. Using the model, we derive an exact expression for the distribution of the number of bursts that contend with an arriving burst. The model is applicable to systems in which each class has an arbitrary burst-length distribution and an arbitrary offset size. We also derive accurate approximate expressions for the burst-blocking probability of premium-class traffic, as well as expressions for the sensitivity of premium-class performance to offset jitter and variations in the arrival rates of each class. In a case study, we find that scaling up a system in terms of the number of wavelengths and the traffic load significantly improves not only the burst-blocking performance of the premium class, but also its sensitivity to lower class traffic variations. We also use the model to dimension and provision the system to guarantee a minimum level of premium-class blocking and premium-class robustness to low-class load variations.

In optical burst switching (OBS) networks, class differentiation and isolation can be achieved by assigning adequately long time offsets between the control packet and payload of high-class bursts. While it has been recognized that the length distribution of low-class bursts plays a role in determining the size of the offsets required, there have been no studies on the effect of other factors that may also be significant. In this paper we examine the effect of the ratio of the arrival rates of low-class and high-class traffic on the level of isolation achieved in OBS networks with quality of service offsets. We show that the level of isolation in the network depends on the arrival rate of low-class traffic, especially when the amount of low-class and high-class traffic in the system is comparable. When we vary the ratio between low and high-class arrival rates from 0.1 to 10, an additional offset of three times the mean low-class burst length is required to achieve the same level of isolation. These results imply that it is important for researchers and network designers to take into account the amount of low-class traffic in the network when provisioning offsets for class differentiation in OBS networks.

This paper presents an analytical model that evaluates the blocking probability of each service class in optical burst-switching networks. The model is applicable to systems with arbitrary burst length distributions and arbitrary-sized QoS header offsets. Thus, unlike previous models, it is applicable to the design and study of networks with a wide range of traffic characteristics, including systems in which higher classes are not necessarily isolated from lower classes and systems in which the conservation law does not necessarily hold. We derive explicit expressions for blocking probability both the cases of constant burst lengths and exponentially distributed burst lengths and verify the model's accuracy through simulation. We show the model to be accurate for a number of different traffic loads and class priorities. For an OBS system with two classes and a 1:10 ratio of high-priority to low-priority traffic, our model is able to predict accurately the blocking probability for each class, whereas the predictions from a model that assumes isolation deviates by as much as an order of magnitude from the simulation results for the higher priority class.

In this paper, we examine the use of limited tunability in reconfigurable optical add-drop multiplexers (L-ROADM). L-ROADMs can add or drop from only a subset of adjacent wavelengths on the network and are less costly than fully-reconfigurable optical add-drop multiplexers (F-ROADMs). We quantify the trade-off between tuning range and the number of F-ROADMs that can be replaced by L-ROADMs without sacrificing the set of connections that can be established. For the limited-add and drop case, an analytical solution for the band size is found. For the limited-add or limited-drop case, a nearly linear relationship was found between the size of the band and the number of L-ROADMs required, and the number of additional wavelengths required never exceeded 20%. For example, if half of the nodes in the ring were equipped with L-ROADMs that operated on 50% of the total spectrum, full connectivity could still be achieved by employing as few as 7% extra wavelengths.

The utilization, defined as the throughput normalized to the time-domain processing gain, is used as metric to compare the performance of optical code-division multiple-access (WDMA) local-area networks. The impact of shot and thermal noise, as well as multiple-access interference (MAI) for the O-CDMA system, is taken into account. We find that a low offered loads or low received power levels, the WDMA system outperforms the O-CDMA system by a wide margin; however, at higher offered loads and when sufficient optical power is available at the receiver, the O-CDMA scheme offers a performance advantage. The results suggest that an adaptive system, able to switch between WDMA and O-CDMA, would make the best use of the fiber-optic channel.

Prime codes are excellent candidates for use in multiwavelength optical code-division multiple-access (O-CDMA) systems. We show that in an O-CDMA system using two-dimensional single-pulse-per-row codes, a single choice of the number of wavelength channels can. accommodate different numbers of users with maximal spectral efficiency. The optimum single-user-detection spectral efficiency of the system can be reached using AND detection. A fixed-hardware network can readily be adapted in response to changes in the number of users and traffic load.

An agile, transparent optical network is emerging. This paper enumerates the functions that will be needed at nodes in order to add transparency and agility to the network while robustly assuring optical fibre channel performance. One key requirement will be scalable integration of multiple functions on a platform. The paper presents recent results along one strategic axis for integration - the use of functionalized self-organized photonic crystals and heterostructures thereof to control the flow and features of light.

We examine the properties and applications of a general class of multi-wavelength optical code-division multiple access codes in high-speed optical local area networks. We demonstrate that multiple pulse per row codes with optimum threshold detection admit maximization of the number of simultaneous users and spectral efficiency. The code design problem is also greatly simplified when the optimum threshold detection is employed. Based on our results, we propose a simple and low-cost receiver to enable true real-time network optimization.

Temporal skewing, a hitherto unexamined effect of chromatic dispersion on multiwavelength optical code-division multiple-access (MW-O-CDMA) systems, is explored through simulation of realistic networks. In addition, two novel methods for combating the deleterious effects of temporal skewing are proposed: optimum threshold detection and code-pattern preskewing. It is shown that for a practical system configuration with optimum threshold detection, MW-O-CDMA networks can maintain design bit error rate performance within an order of magnitude for link lengths of up to approximately 500 m. Our results show that the effect merits careful attention in the design and realization of any MW-O-CDMA system based on two-dimensional codes. Code pattern preskewing presents the possibility of completely eradicating the impact of temporal skewing.

A universal scheme is developed to reproduce the effect of bipolar signaling in a unipolar optical channel. Without any hardware modification, the electro-optic/opto-electronic (EOE) system proposed enables the transmission of any bipolar code sequence through the unipolar optical channel while maintaining the correlation properties of the codes. The bit error rate performance of the EOE system is shown to be identical to that of an optimal baseband bipolar phase shift keying code-division multiple access (CDMA) system. Hence, the maximum spectral efficiency of an incoherent optical CDMA system using bipolar codes is the same as that of an optimal bipolar baseband CDMA system.

We combine forward error correction and dynamic threshold optimization to achieve a dramatic increase in the spectrum utilization of multi-wavelength optical code division multiple access (MW-OCDMA) local area networks.