Super-high temperature sensitivity of long-period gratings in B/Ge co-doped fiber

Long period fiber grating (LPFG) can be used as active gain controlling device in EDFA. However, LPFGs fabricated in the standard telecom fiber only have a typical temperature sensitivity of 3-10nm/100°C, which may not be sufficient for implementing tuneable filters capable of wide tuning range and high tuning efficiency. In this paper, we report a theoretical and experimental investigation of thermal properties of LPFGs fabricated in B/Ge co-doped optical fiber. We have found that the temperature sensitivity of the LPFGs in the B/Ge fiber is considerably increased compared with those produced in the standard fiber. The LPFGs written in the B/Ge fiber have achieved, on average, one order of magnitude higher sensitivity than that of the LPFGs produced in the standard telecom fiber. We have also identified that the thermal response of LPFG is strongly dependent on the order of the coupled resonant cladding mode. The maximum sensitivity of 1.75nm/°C achieved by the 10th cladding mode of the 240μm LPFG is nearly 24 times that of the minimum value (0.075nm/C) exhibited by the 30th mode of the 34μm LPFG. Such devices may lead to high-efficiency and low-cost thermal/electrical tunable loss filters or sensors with extremely high temperature resolution.

Picosecond pulse amplification up to a peak power of 42 W by a quantum-dot tapered optical amplifier and a mode-locked laser emitting at 1.26 μm

We experimentally study the generation and amplification of stable picosecond-short optical pulses by a master oscillator power-amplifier configuration consisting of a monolithic quantum-dot-based gain-guided tapered laser and amplifier emitting at 1.26 μm without pulse compression, external cavity, gain-or Q-switched operation. We report a peak power of 42 W and a figure-of-merit for second-order nonlinear imaging of 38.5 W2 at a repetition rate of 16 GHz and an associated pulse width of 1.37 ps.

Compact sensors based on cascaded single-mode-multimode-single-mode fiber structures

Concatenated single-mode-multimode-single-mode (SMS) structures are demonstrated as functional sensing platforms. The devices are fabricated by periodically inserting micrometric sections of multimode optical fiber (MMF) in a single-mode fiber (SMF). The periodic change of the core diameter produces a single strong resonant transmission notch, tunable in the wavelength range from 1200 to 1600 nm. It was found that the position of the notch changed with temperature and refractive index. The devices introduced here are highly compact (length less than 5 mm), simple to fabricate and robust; hence, they are adequate for diverse sensing applications. © 2013 The Japan Society of Applied Physics.

Flanerie: A post -reflexive mode of urban research inquiry

Urban ethnographic studies in social science usually proceed from within a pre-figured research framework that guides field activity and filters what is considered see-worthy and study-worthy to those phenomena which are in some way necessary for the fulfillment of specific research agendas. Reliance on formalized ethnographic research methods to explore the city, and the reflex to intellectualize that which is observed by using them, frequently leads to the neglect of much of what comprises the ethnographic richness of city-life through an overly-contemplative intellectuocentrism. ^ Flânerie, an avocational, proto-sociological ethnographic mode of urban exploration and observation originating in early-nineteenth-century Paris, is representative of an alternate approach to exploration and study of the city—one less subject to a compressed horizon of ethnographic experience. It affords insight into a broad range of phenomena taking place in interstitial urban space and time which often escape the instrumentally overly-determined gaze of professionalist research inquiry. ^ The present work addressed the fact that the concept of flânerie , though occasionally invoked in discussions of methodology in urban sociology and cultural studies, had not been subjected to a systematic attempt to relate it to the research methods of these disciplines. Though as a typological figure the flâneur is thought to have disappeared long ago, practices represented by the concept continue to be engaged in by persons inside and outside of academia who approach city life and social science with a perspective somewhat transcending the reified dichotomies of private-professional and art-and-science. This study transferred aspects of the experience of the city embodied in the flâneur to the realm of ethnographic urban studies and the building of grounded social theory, finding flânerie to be crucial to the development of a refined knowledge of urban life, and an important means by which previously overlooked social phenomena and silenced avenues of research inquiry might be exposed. ^ The conclusions reached found a “flâneuristic” approach to the city as being beneficial to research practice and the vitalization of urban ethnographic inquiry, by affording opportunities for exposure to, and immersion within, an expanded range of quotidian social phenomena that have frequently escaped the academicist gaze. ^

On the group delay statistics of few-mode fibres with intermediate linear mode coupling

We report an investigation on the statistics of group delay for few-mode fibres operating in the weak and strong linear coupling regimes as well as in the intermediate coupling regime. A single expression linking the standard deviation of the group delay spread to the fibre linear mode coupling is validated for any coupling regime, considering up to six linearly polarized guided modes. Furthermore, the study of the probability density function of the group delays allowed deriving and validating an analytical estimation for the maximum group delay spread as a function of linear mode coupling.

Near-field coupling and resonant cavity modes in plasmonic nanorod metamaterials

Plasmonic resonant cavities are capable of confining light at the nanoscale, resulting in both enhanced local electromagnetic fields and lower mode volumes. However, conventional plasmonic resonant cavities possess large Ohmic losses at metal-dielectric interfaces. Plasmonic near-field coupling plays a key role in a design of photonic components based on the resonant cavities because of the possibility to reduce losses. Here, we study the plasmonic near-field coupling in the silver nanorod metamaterials treated as resonant nanostructured optical cavities. Reflectance measurements reveal the existence of multiple resonance modes of the nanorod metamaterials, which is consistent with our theoretical analysis. Furthermore, our numerical simulations show that the electric field at the longitudinal resonances forms standing waves in the nanocavities due to the near-field coupling between the adjacent nanorods, and a new hybrid mode emerges due to a coupling between nanorods and a gold-film substrate. We demonstrate that this coupling can be controlled by changing the gap between the silver nanorod array and gold substrate.

Nonlinear optics with cold Rb atoms using tapered optical nanofibres

Optical nanofibres (ONFs) are very thin optical waveguides with sub-wavelength diameters. ONFs have very high evanescent fields and the guided light is confined strongly in the transverse direction. These fibres can be used to achieve strong light-matter interactions. Atoms around the waist of an ONF can be probed by collecting the atomic fluorescence coupling or by measuring the transmission (or the polarisation) of the probe beam sent through it. This thesis presents experiments using ONFs for probing and manipulating laser-cooled 87Rb atoms. As an initial experiment, a single mode ONF was integrated into a magneto-optical trap (MOT) and used for measuring the characteristics of the MOT, such as the loading time and the average temperature of the atom cloud. The effect of a near-resonant probe beam on the local temperature of the cold atoms has been studied. Next, the ONF was used for manipulating the atoms in the evanescent fields region in order to generate nonlinear optical effects. Four-wave mixing, ac Stark effect (Autler-Townes splitting) and electromagnetically induced transparency have been observed at unprecedented ultralow power levels. In another experiment, a few-mode ONF, supporting only the fundamental mode and the first higher order mode group, has been used for studying cold atoms. A higher pumping rate of the atomic fluorescence into the higher order fibreguided modes and more interactions with the surrounding atoms for higher order mode evanescent light, when compared to signals for the fundamental mode, have been identified. The results obtained in the thesis are particularly for a fundamental understanding of light-atom interactions when atoms are near a dielectric surface and also for the development of fibre-based quantum information technologies. Atoms coupled to ONFs could be used for preparing intrinsically fibre-coupled quantum nodes for quantum computing and the studies presented here are significant for a detailed understanding of such a system.

Temperature measurement of cold atoms using transient absorption of a resonant probe through an optical nanofibre

Optical nanofibres are ultrathin optical fibres with a waist diameter typically less than the wavelength of light being guided through them. Cold atoms can couple to the evanescent field of the nanofibre-guided modes and such systems are emerging as promising technologies for the development of atom-photon hybrid quantum devices. Atoms within the evanescent field region of an optical nanofibre can be probed by sending near or on-resonant light through the fibre; however, the probe light can detrimentally affect the properties of the atoms. In this paper, we report on the modification of the local temperature of laser-cooled 87Rb atoms in a magneto-optical trap centred around an optical nanofibre when near-resonant probe light propagates through it. A transient absorption technique has been used to measure the temperature of the affected atoms and temperature variations from 160 μk to 850 μk, for a probe power ranging from 0 to 50 nW, have been observed. This effect could have implications in relation to using optical nanofibres for probing and manipulating cold or ultracold atoms.

Ultra Compact Inline E-Plane Waveguide Bandpass Filters Using Cross Coupling

This paper presents novel ultra-compact waveguide bandpass filters that exhibit pseudo elliptic responses with ability to place transmission zeros on both sides of the passband to form sharp roll offs. The filters contain E plane extracted pole sections cascaded with cross-coupled filtering blocks. Compactness is achieved by the use of evanescent mode sections and closer arranged resonators modified to shrink in size. The filters containing non-resonating nodes are designed by means of the generalized coupling coefficients (GCC) extraction procedure for the cross-coupled filtering blocks and extracted pole sections. We illustrate the performance of the proposed structures through the design examples of a third and a fourth order filters with center frequencies of 9.2 GHz and 10 GHz respectively. The sizes of the proposed structures suitable for fabricating using the low cost E plane waveguide technology are 38% smaller than ones of the E plane extracted pole filter of the same order.

Aniseikonia tests: The role of viewing mode, response bias, and size-color illusions.

PURPOSE To identify the factors responsible for the poor validity of the most common aniseikonia tests, which involve size comparisons of red-green stimuli presented haploscopically. METHODS Aniseikonia was induced by afocal size lenses placed before one eye. Observers compared the sizes of semicircles presented haploscopically via color filters. The main factor under study was viewing mode (free viewing versus short presentations under central fixation). To eliminate response bias, a three-response format allowed observers to respond if the left, the right, or neither semicircle appeared larger than the other. To control decisional (criterion) bias, measurements were taken with the lens-magnified stimulus placed on the left and on the right. To control for size-color illusions, measurements were made with color filters in both arrangements before the eyes and under binocular vision (without color filters). RESULTS Free viewing resulted in a systematic underestimation of lens-induced aniseikonia that was absent with short presentations. Significant size-color illusions and decisional biases were found that would be mistaken for aniseikonia unless appropriate action is taken. CONCLUSIONS To improve their validity, aniseikonia tests should use short presentations and include control conditions to prevent contamination from decisional/response biases. If anaglyphs are used, presence of size-color illusions must be checked for. TRANSLATIONAL RELEVANCE We identified optimal conditions for administration of aniseikonia tests and appropriate action for differential diagnosis of aniseikonia in the presence of response biases or size-color illusions. Our study has clinical implications for aniseikonia management.

Mode locking in a spatially extended neuron model: active soma and compartmental tree

Understanding the mode-locked response of excitable systems to periodic forcing has important applications in neuroscience. For example it is known that spatially extended place cells in the hippocampus are driven by the theta rhythm to generate a code conveying information about spatial location. Thus it is important to explore the role of neuronal dendrites in generating the response to periodic current injection. In this paper we pursue this using a compartmental model, with linear dynamics for each compartment, coupled to an active soma model that generates action potentials. By working with the piece-wise linear McKean model for the soma we show how the response of the whole neuron model (soma and dendrites) can be written in closed form. We exploit this to construct a stroboscopic map describing the response of the spatially extended model to periodic forcing. A linear stability analysis of this map, together with a careful treatment of the non-differentiability of the soma model, allows us to construct the Arnol'd tongue structure for 1:q states (one action potential for q cycles of forcing). Importantly we show how the presence of quasi-active membrane in the dendrites can influence the shape of tongues. Direct numerical simulations confirm our theory and further indicate that resonant dendritic membrane can enlarge the windows in parameter space for chaotic behavior. These simulations also show that the spatially extended neuron model responds differently to global as opposed to point forcing. In the former case spatio-temporal patterns of activity within an Arnol'd tongue are standing waves, whilst in the latter they are traveling waves.