Low Gain Values Research Articles

Spectral engineering of semiconductor Fabry-Perot laser cavities in the weakly and strongly perturbed regimes

By inserting index perturbations at certain positions along a semiconductor Fabry-Perot laser cavity the threshold gain for one or several of the longitudinal cavity modes can be selectively lowered to facilitate, e.g., single-mode or two-color operation. Previous design methods were limited to a fairly small number of perturbations, leading to only weakly perturbed cavities and thus a limited freedom in tailoring the spectral properties of the laser. In our approach we fully account for all multiple-reflection events and use a search space that permits any distribution of the locations and lengths of the perturbations. We are therefore able to design cavities with almost arbitrary spectral properties with very low threshold gain values for, e.g., the lasing modes of a two-color cavity. Constraining the design by reducing the geometrical freedom, which can be used to increase the smallest feature size to simplify fabrication, we seamlessly approach the weakly perturbed regime while maintaining much of the freedom for spectral engineering.

Efeito das condições da desidratação osmótica na qualidade de passas de caju-do-cerrado

A metodologia de superfície de resposta foi usada para determinar as melhores condições de processamento de acordo com a máxima perda de peso e de umidade, baixo valor de atividade de água e incorporação de sólidos e maior concentração de vitamina C na elaboração de passas de caju-do-cerrado, utilizando-se a desidratação osmótica como pré-tratamento. O tempo de tratamento osmótico (2 a 4 horas), a temperatura (30 a 50 °C) e a concentração da solução osmótica (40 a 60 °Brix) foram as variáveis investigadas para as respostas: perda de peso, perda de umidade, incorporação de sólidos, atividade de água e teor de vitamina C no produto final. Os experimentos foram conduzidos conforme Delineamento Central Composto Rotacional com 19 tratamentos, incluindo pontos axiais e centrais. Para cada resposta, modelos de segunda ordem polinomial foram desenvolvidos usando-se análise de regressão múltipla linear. Foi realizado teste sensorial com 33 provadores para observar a aceitação dos produtos com qualidades tecnológicas e nutricionais adequadas, sendo que a passa de caju-do-cerrado processada com solução osmótica de 40 °Brix, 50 °C, durante quatro horas, apresentou os melhores resultados.

Suitability of Short-Period Sensors for Retrieving Reliable H/V Peaks for Frequencies Less Than 1 Hz

Using three different short-period electromagnetic sensors with resonance frequencies of 1 Hz (Mark L4C-3D), 2 Hz (Mark L-22D), and 4.5 Hz (I/O SM-6), coupled with three digital acquisition system, the portable data acquisition system (PDAS) Teledyne Geotech, the refraction technology (REFTEK) 72A, and the Earth Data Logger PR6-24 (EDL), the effect of the seismic instruments on the horizontal-to-vertical spectral ratio (H/V) using seismic noise for frequencies less than 1 Hz has been evaluated. For all possible sensors–acquisition system pairs, the background seismic signal and instrumental self-noise power spectral densities have been calculated and compared. The results obtained when coupling the short-period sensors with different acquisition systems show that the performance of the considered instruments at frequencies <1 Hz strongly depends upon the sensor–acquisition system combination and the gain used, with the best performance obtained for sensors with the lowest resonance frequency. For all acquisition systems, it was possible to retrieve correctly the H/V peak down to 0.1–0.2 Hz by using a high gain and a 1-Hz sensor. In contrast, biased H/V spectral ratios were retrieved when low-gain values were considered. Particular care is required when using 4.5-Hz sensors, because they may not even allow the fundamental resonance frequency peak to be reproduced.

Simultaneous amplification by Er ions and SRS in an Er-doped germano-silica fiber

A flat signal gain over in the entire C- and L-bands by erbium (Er) ions' radiative transition and stimulated Raman scattering in an Er-doped germano-silica fiber can be obtained if proper values of the concentration of Er and background loss in a fiber core are obtained during the fiber fabrication process. The optimized conditions for the flat C- and L-band gain are analyzed as functions of Er concentrations. Even for a low-gain value provided by a germano-silica core fiber with a low Er concentration and an optimum fiber length, a relatively low pump is required to obtain the flat gain band.

Ocular responses to head rotations during mirror viewing.

The gain of the human vestibuloocular reflex (VOR) is influenced by the proximity of the object of regard. In six human subjects, we measured the eye rotations induced by passive, sinusoidal, horizontal head rotations at 2.0 Hz during binocular fixation of a stationary far target at 7 m; a stationary target close to the subject's near point of fixation (<15 cm); and the bridge of the subject's own nose, viewed through a mirror positioned so that, for each subject, the angle of vergence was similar to that during viewing of the near target. The median gain of compensatory eye movements for the group of subjects during far viewing was 0.99 (range 0.80-1.04), during near viewing was 1.21 (range 0.88-1.47), and during mirror viewing was 0.85 (range 0.71-1.01). The gain during near and mirror viewing was significantly different for each subject (P < 0.001) even though the vergence angles were similar. The lower gain values during mirror viewing can be attributed to the geometric relationship between the head rotation, the position of the eyes in the head, and the movement of the subject's virtual image in the mirror. To determine whether visually mediated eye movements were responsible for the observed gain values, we conducted a control experiment in which subjects were rotated using a sum-of-sines stimulus that minimized the effects of predictive visual tracking; differences of gain values between near- and mirror-viewing conditions were similar to those during rotation at 2 Hz. We conclude that, in these experiments, target proximity and vergence angle were not the key determinants of gain of the visuo-vestibular response during head rotation while viewing a near target but that contextual cues from motion vision were more important in generating the appropriate response.

A 200-Ms/s 10-mW switched-capacitor filter in 0.5-/spl mu/m CMOS technology

Wideband amplifiers with low but precisely known dc gain allow the achievement of accurate infinite impulse response switched-capacitor (SC) filters operating at very high sampling frequencies. The low and precise opamp gain value is taken into account while sizing the capacitors (precise opamp gain (FOG) approach), so that no idle phase is required for amplitude error compensation and double-sampling technique can be implemented. In a 0.5-/spl mu/m standard CMOS technology with 3.3-V power supply, an opamp is designed which exhibits a settling time of about 3 ns (for 0.1% settling accuracy) in a closed-loop configuration with input, feedback, and load capacitors of 0.5 pF, white the slew rate is 1 V/ns. The open-loop dc gain of the amplifier is set to the value of 80 (38 dB) by a gain-control closed loop, which guarantees an accuracy of /spl plusmn/2%. The proposed solution is validated by experimental results from a 200-Ms/s SC filter. From a single 3.3-V supply the filter consumes 10 mW (excluding clock generation) and exhibits a -40 dB total harmonic distortion for a 2-V/sub pp/ signal amplitude at 4 MHz, achieving a 62-dB dynamic range.

SFE16, a low noise front-end integrated circuit dedicated to the read-out of large Micromegas detectors

A front-end BiCMOS ASIC was specially developed for the Micromegas detectors to be used in the Small Angle Tracker of the COMPASS experiment at CERN. Each of the 16 channels of this integrated circuit contains a low noise preamplifier with a 100 ns peaking time filter and a discriminator driving a low-level differential digital buffer. The design of the preamplifier and the choice of the shaping have been tuned to the detector signal shape in order to allow the operation of Micromegas even for very low multiplication gain values. Noise measurements show an equivalent noise charge of less than 1500 e-rms for a detector capacitance of 40 pF. The measured performances of this ASIC associated or not with the detector are fully described in this paper.

Calculation of differential gain and linewidth enhancement factor in 980-nm InGaAs vertical cavity surface-emitting lasers

The optimization of differential gain and linewidth enhancement factor within VCSEL's is required to achieve improved performance under dynamic conditions. In this paper, the differential gain and linewidth enhancement factor in an InGaAs-GaAs VCSEL have been calculated over a range of carrier densities and at various wavelengths across the gain spectrum. The results show the importance of low threshold gain values in achieving high differential gain/low linewidth enhancement factor. By temperature tuning the lasing wavelength to shorter wavelengths than the gain peak, values of /spl alpha//sub H/ as low as 0.6 can be achieved.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Biological study on the effect of supplementing wheat flour with fish protein concentrate

The protein quality of 4 Egyptian pastries made from different combinations of flour and fish protein concentrate (0%, 4% and 6%) was measured by rat growth study. Weight gain, feed consumption and PER were determined. Weight gain of rats fed cottage cheese crescents was higher than that fed date bars followed by that fed brouche, then rats fed salty sticks. Feed consumed by rats fed with the corresponding commercial pastries was low except in date bars group. Commercial pastries led to low weight gain and low PER values. Supplementing wheat flour with 6% fish protein concentrate gave the highest weight gain and the highest PER values.

Spatially distributed gains in semiconductor lasers

A careful analysis of the gain using the amplified spontaneous emission in semiconductor lasers concludes that there is a spatial distribution of the gain. A twofold distribution is observed in the planes parallel and perpendicular to the junction with different decreasing variations from the laser axis to the edge of the radiation lobe. Collecting the entire emitted light beam gives usually too low gain values. Gain measurements restricted to the region around the laser beam axis lead to an improvement of the gain value determination being within 96% of the actual value determined by the laser threshold.

Diagnostic Value of Velocity-Step Responses

For the nystagmic (after-)response in each direction it was evaluated in 618 patients whether the gain (initial velocity) and the time constant were normal, significantly low or high, thus constituting a specific 'response pattern'. In a number of diagnostic categories, 65% of the response patterns were 'typical', i.e., they were found in significantly high frequency. Symmetrical response patterns were predominant. Short time constant(s) and/or low gain values in either or both directions were found in labyrinthine defects. High gain values were found in otosclerosis after footplate surgery, in multiple sclerosis, hyperventilation syndrome, posterior fossa tumour and spasmodic torticollis. In those categories, except otosclerosis and posterior fossa tumour, also significantly high values of the time constant were found, as well as in the category of patients with exposed tympanic cavity and dizziness. A significant directional preponderance indicating the defective side was found for the low-frequency gain (i.e., the product of initial velocity and time constant) in 48% of the patients with unilateral canal paralysis. It is concluded that the velocity-step test is a powerful diagnostic tool in vestibular examination.

LONGITUDINAL MODES IN A HIGH-GAIN LASER

In lasers employing high-gain narrow-linewidth transitions the theory predicts major departures of the mode-splitting frequencies from their low-gain values as well as a new type of mode splitting. The first of these effects consisting of a reduction by a factor of 2.5 of the mode splitting in a xenon 3.51-μ laser is observed experimentally.

A Unilateral Parametric Amplifier

A theoretical investigation of a unilateral parametric amplifier using two varactor diodes indicates an improvement of unilateral stability over already existing types. A circuit is suggested which uses lower sideband idler energy for achieving forward gain and upper side-band energy to obtain substantial reverse loss. The phases of the applied signals and of the pump at the two varactors have to be 90/spl deg/ out of phase to achieve unilateral operation. Numerical evaluation of the theoretical results for a signal frequency at 4.0 GHz and a pump frequency at 12.0 GHz, assuming a diode junction capacitance of C/sub j/ = 0.4 pF and a bulk resistance of R/sub s/ = 2/spl Omega/ was done for several pump power levels. For 14 dB maximum forward gain, the 3 dB bandwidth of the gain versus frequency characteristic of the unilateral amplifier is about 18 percent smaller than that of the reflection type amplifier. The maximum reverse loss for these conditions is 7.3 dB. For lower forward gain the backward loss increases relatively until for very low gain values (about 1 dB) the amplifier is unconditionally stable, i.e., the backward loss is larger than the forward gain. The theoretical noise figure is about 1.95 dB at signal center frequency for 14 dB forward gain and, for /spl plusmn/80 MHz from the center frequency, only 0.1 dB higher than for the reflection type amplifier.