Low-gain System Research Articles

Investigation of a diode-pumped Ti:sapphire laser modelocked using carbon nanotubes

We report a diode-pumped femtosecond Ti:sapphire laser in which a suspension of single-walled carbon nanotubes spin-coated onto a cavity mirror is the saturable absorber element. Laser performance is presented for single-diode pumping at 462 nm with 51-fs pulses being achieved with an output power of 27 mW. The laser exhibits stable operation with a relative intensity noise of 0.26% and a pulse repetition frequency of 79.24 MHz. Measurements reporting saturable and non-saturable losses each of ∼0.1% illustrate that this saturable absorber element is particularly appropriate for use in a low gain system such as a diode-pumped Ti:sapphire laser.

Review on Ultra-Wideband Phased Array Antennas

Ultra-wideband (UWB) phased array antennas are essential for radar, communication, and Electronic Warfare (EW) applications. Systems can operate over wide-band coverage. The high gain systems require wide-band, wide-angle scanning, good radiation pattern, and return loss over a multi-octave bandwidth. This review discusses the problems present in the UWB phased array system, which include mutual coupling, grating lobe rejection, active return loss, gain fluctuation with scanning, and multiple beam generation. In this, the aperture size of the array is limited by the overall gain requirement of the system. On the other hand, low gain systems require a wide-band with a good radiation pattern and axial ratio. Here the matching over wide-band is simpler as it depends on broadband balun configuration. This system has limited bandwidth over scan angles. These antennas are more preferred as wearable antennas in communication applications. This paper mainly discusses the existing UWB phased array antennas such as Vivaldi Arrays, All-metal Arrays, Spiral Arrays, Sinuous Arrays, and Coupled Arrays. This review paper presents a correct direction for selecting the type of antenna in terms of impedance bandwidth, gain, and materials influencing antenna performance, suited for different applications.

Analytic theory for lasers based on stimulated polariton scattering

We present a model for predicting the development of Stokes and terahertz (THz) fields undergoing stimulated polariton scattering. The model is valid for finite beams, avoiding the most limiting assumption of the leading model to date; our model makes significantly different predictions for the narrow pump beams and low-gain systems now commonly being studied. The model accounts for the build-up of the THz field across finite pump beams as well as naturally extending that propagation away from the pumped region, a key requirement for understanding the effectiveness of real experimental geometries.

Predicting the evolutionary dynamic behavior of a laser with injected signal using Lyapunov exponents

We show that the role of the Lyapunov exponents can be extended beyond the customary local instability, such as limit cycle behavior, to include its use as an evolutionary predictor of the dynamics of a laser with injected signal (LIS). Numerical studies of LIS reveal that as a function of the input-signal strength the evolution of two nonzero Lyapunov exponents (generally equal) distinctively predicts the evolutionary trend of the fundamental frequency of the laser output signal (an important dynamic characteristic of the LIS) even with the presence of some noise. This globally predictive behavior of the Lyapunov exponents includes also the dynamic behavior of the individual coexisting attractors. Different coexisting attractors of LIS and configurations of Lyapunov exponents for both individual attractors and the global system are reported. Two LIS case studies are considered: (I) a high-gain system with a rich history of nonlinear behavior but not experimentally accessible, and (II) a low-gain system that has complex dynamics and is experimentally accessible for Class B lasers. Universality arguments support the thesis that these different configurations and the extended role of the Lyapunov exponents as an evolutionary predictor of the dynamics will be observed in other nonlinear, dynamic dissipative systems as well.

Integrating economic gain in biosocial systems

AbstractThe concept of gain and profit is introduced into ecology as a way of summarizing strategies of biological and social structures. High gain systems can be predicted by flux as they take in fuel at a rate. Low gain systems must refine low‐quality materials in order to acquire fuel. Low gain systems are predictable from their plans and coded behaviour. Changes from high to low gain mode and vice versa represent a reordering of a hierarchy over time. We use a catastrophe pleat as a way to model high gain collapse. Collapse is avoided in low gain by planning. At first, planning enlarges the system through economies of scale, but eventually resources are of such low quality that gathering and refining begins to limit size. We also model shifts from high to low gain with a response surface of resource used against adaptation. There are peaks of high and low gain adaptation with which we analyse termite evolution. Positioning systems on the surface is complicated by valid alternative interpretations of system behaviour. Each low gain phase starts with a burst of high gain so it is possible to reinterpret successive moves to the lower gain as fractal shifts that link successive response surfaces. We propose mathematical protocols for capturing the response surfaces. Copyright © 2010 John Wiley & Sons, Ltd.

Sustainability and livability in the 2007 banking crisis: A failed transition to a low gain system

AbstractThere is a contrast between the role of complexity in enabling value creation and market growth and its role in limiting sustainability. This contrast can be understood by using the concept of low gain systems, which are relatively elaborate and sustainable, and the concept of high gain systems which are concerned with the immediacy of actually living rather than longer‐term sustaining. This paper analyses the recent banking crisis to develop a framework that weaves together the many suggestions that have been made to fix the banking system and to look for a root cause. The root cause of the banking crisis was in the increased complexity of the supply chain that linked more borrowers to more investors. This complexity enabled the aggregation and customization of debt product for specific investors, which grew the market. But it was not elaborate enough to preserve information that was required to manage the quality of these debt products. So the negative feedback of toxic debts poisoned the system in a high gain manner rather than being filtered out in a low gain manner. This was allowed to happen because of missing elements at the global and national levels of the regulation system which did not spot this filtering problem or were unable to constrain the lower levels of the regulation system to options that would fix it. The implications for regulating future system elaboration attempts are also discussed. Copyright © 2010 John Wiley & Sons, Ltd.

Fostering innovation in medical technology with hierarchy theory: Narratives on emergent clinical solutions

AbstractInnovating with the goal of conquering disease and injury is one of humanity's most admirable activities. With the variability of patients and diseases, medical innovation often involves complex interactions between the physicians in clinical practice and the rapid technological change from commercial firms. We apply aspects of interpretative hierarchy theory dealing with self‐reflective processes, particularly observation and description, to analyse these complex and dynamic relationships that foster innovation in medical technology. This systemic approach fixes certain parameters that help reduce complexity, (i) the notion of gain to adequately describe the change of observed technological hierarchies over time, and (ii) a systems description that allows clinical innovation to emerge as narratives. Our view of innovation in medical technology as an outcome of a high and low gain system finds its empirical illustration through the emergence of diagnostic solutions in medical imaging in the past decade. Finally, our research suggests epistemological and practical contributions to support the sound technological innovation in clinical practice with emphasis on selection criteria and holism for different stakeholders including physicians, firm managers and policymakers. Copyright © 2010 John Wiley & Sons, Ltd.

Effects of including a diffraction term into Rigrod theory for a continuous-wave laser

Multimode, low-gain continuous-wave lasers are often subject to having intracavity apertures that create diffractive losses inside the optical resonator. For very low-gain systems with short gain lengths, highly reflective mirrors are required to obtain laser oscillation. The Rigrod theory was modified to include a diffractive loss term and comparisons with experimental data show that the intracavity diffractive losses, while small in magnitude, can play a significant role for these low-gain cases with high mirror reflectivities.

Laser action in strongly scattering rare-earth-metal-doped dielectric nanophosphors

We report continuous-wave laser action in two oxide nanopowders doped with ${\mathrm{Ce}}^{3+}$ and ${\mathrm{Pr}}^{3+}$ ions. To our knowledge, these are the first observations of stimulated emission from electrically pumped random media in which scattering and distributed feedback scale lengths are much shorter than a wavelength. The continuous nature of the laser emission in small volumes (on the order of a cubic wavelength) of these low gain systems, and the absence of coherent speckle, provide compelling evidence that these results are mediated by strong Anderson localization of light.

Detection and removal of surface defects in optical materials

Numerous insulating materials are used as substrates for optical waveguide versions of lasers, second harmonic generation, electro-optics and photonic devices but, independent of the production methods, the losses in the guides are considerably worse than those of bulk material. It is therefore essential to overcome this major problem, not least because success would allow a much wider range of materials to be included in waveguide applications, (e.g. by developing low gain systems such as tuneable laser waveguides). An obvious source of the loss occurs in the cutting and polishing of the surface since, although the outer layer may be optically flat, the subsurface is highly strained and has a high dislocation density to depths of several microns (i.e. precisely in the depth regime where the waveguides are formed). Such dislocation probelms can be revealed and, in the case of Nd : YAG, improvements introduced by amorphisation and chemical etching of the surface, to generate a damage free surface layer. Once the damaged YAG layer was removed cathodoluminecence (CL) intensity increased by 250%, SHG effects were noted, and for waveguides photoluminescence increased by 40% and transmission was improved. The method employed may be applied much more widely (e.g. in garnets, LiNbO3, Quartz and phosphates).

Review of radio frequency beamforming techniques for scanned and multiple beam antennas

The growing complexity of communication and surveillance systems means that there is an increasing need for antennas with multiple beam capability. Several technologies such as radio frequency (RF), intermediate frequency (IF), digital and optical are available to meet this need but RF beamforming has the advantage of simplicity and a good capability for many applications. This state of the art review of RF beamforming methods for multiple beam antennas aims to introduce the topic and to collate and classify types, giving a critical assessment of comparative performance. The fundamental relationship between the number of active control elements and the number of beams is taken as a measure of beamformer performance and this also indicates that, for a limited number of active elements, high gain antennas with narrow beams will have limited scan range whilst low gain systems have a wide angle capability. This deduction is reinforced by the practical results collected at the end of the review. The beamformer types covered include lens based beamformers such as the Ruze, Rotman, R-KR and nonuniform refractive index types together with objective lenses with separate feed arrays. Reflector based multiple beam antennas based on zero, one or two Fourier transform properties are followed by circuit types including the Blass and Butler matrices. The wide range of information presented is clarified by a summary section giving an indication of comparative performance levels. The very diversity of RF beamformer types and performance are seen to attest the importance of the topic area for current and future antenna systems and point the way to further innovation and development in this very significant field.

Optical Gain and Saturation Considerations of Organic Dyes

A theoretical analysis of optical gain and saturation of organic dye lasers is presented. An analysis based on a solution o f the laser rate equations establishes a convenient method of optical gain measurement by internal fluorescence probing for low-gain system s, such as vapors of organic dyes. The concomitant limitations of high-gain system s (e.g. organic dyes in liquid solutions) of small-signal gain due to amplified spontaneous emission (ASE) and problems associated with gain saturation at high signal inputs are discussed in detail.

Transition selection with adjustable outcoupling for a laser device applied to CO<inf>2</inf>

In this paper we discuss the possibilities and realization in high-power systems of a narrow-band reflector with adjustable high reflectivity and with continuous tuning over a large frequency region. We describe a design for both low- and high-gain systems. For the experimental realization of a low-gain system we used a CO 2 laser and studied a continuous range of 65 transitions of the 00\deg1-02\deg0 and 13 transitions of the 0111-1110 vibrational band. With a cavity containing a dc discharge 130 cm long, for each band we found a continuous range of single oscillating transitions with output powers up to 15 W. Finally, we propose an experimental solution for continuous tuning of a high-gain system as, for instance, the central part of the 00\deg1-10\deg0 vibrational band of CO 2 .