Adaptive radiation focusing systems with local optical feedback loop

Moscow State University. Translated from Izvestiya Vysshykh Uchebnykh Zavedenii, Radiofizika, Vol 33, No. 11, pp. 1272–1277, November, 1990.     

Adaptive wave-front correction by means of all-optical feedback interferometry

A novel adaptive wave-front correction system based on an all-optical feedback interferometer is described. In this system the two-dimensional output fringe intensity from a Mach-Zehnder interferometer with large radial shear is optically fed back to an optically addressed phase-only liquid-crystal spatial light modulator. Consequently, without a separate aberration-free reference wave, the modulator phase approximates the conjugate of the interferometer phase that is directly related to the phase of the input aberrated wave front, so this system is applicable in adaptive optics. We successfully achieved real-time correction of aberrated wave fronts: A diffraction pattern that was seriously distorted because of aberrations was transformed into a diffraction-limited spot immediately after the feedback loop was closed.     

Resonance fluorescence of a two-level atom in a feedback loop

The resonance fluorescence spectrum of a two-level atom in a classical light field, whose phase is changed by a feedback system by π after the detection of each spontaneous photon, has been calculated. The spectrum resembles the well-known Rautian-Mollow triplet. However, the sidebands become sharply asymmetric at a nonzero frequency detuning of the light field from the atomic resonance transition. In addition, the spectrum retains the pronounced triplet shape even beyond the secular approximation, when the sidebands of the Rautian-Mollow structure become unresolved in the absence of the feedback.     

Resonance fluorescence statistics of a two-level atom in feedback loop

Statistics of photoemission events into the components of a triplet of resonance fluorescence of a two-level atom in a classical optical field, the phase of which changes by π upon the detection of spontaneous photon, is compared with the case when the feedback is absent. In contrast to the known problem on the statistics of resonance fluorescence of a two-level atom, a grouping of photocounts should be observed in each separate side component of the triplet. Anticorrelation of photoemission into side components is also predicted, and the type of correlations between the emission into the central component of the triplet and into any of the side ones can be controlled by varying the detuning of the radiation frequency from the resonant value.     

Adaptive tuning of feedback gain in time-delayed feedback control

We demonstrate that time-delayed feedback control can be improved by adaptively tuning the feedback gain. This adaptive controller is applied to the stabilization of an unstable fixed point and an unstable periodic orbit embedded in a chaotic attractor. The adaptation algorithm is constructed using the speed-gradient method of control theory. Our computer simulations show that the adaptation algorithm can find an appropriate value of the feedback gain for single and multiple delays. Furthermore, we show that our method is robust to noise and different initial conditions.     

Statistics of resonance fluorescence of a pair of atoms in a feedback loop

The statistics of photoemission events of a pair of closely spaced two-level atoms is calculated in a classical light field whose phase is changed by π after the detection of each spontaneous photon. This statistics is compared with the statistics in the case when the feedback is missing. In both cases, one can observe noticeable antibunching of photons in the range of parameters where no antibunching is observed in a single-atom system. The feedback substantially increases the antibunching. This effect manifests itself more strongly in relatively weak fields and for considerable frequency detunings.     

Aberration correction using a multi-segment mirror with feedback interferometry

A novel real-time aberration correction system using feedback interferometry and a multi-segment mirror for the wavefront corrector is described. The result is a simple and high-speed adaptive optics system demonstrating that feedback interferometry may potentially be an alternative method to conventional aberration correction techniques. Its high-speed processing and simplicity makes real-time correction practical. Our results show that the system can respond up to 37 Hz for sinusoidal phase variations with amplitude of one wavelength and improves Strehl ratio significantly.     

Robustness of the bistable behavior of a biological signaling feedback loop

Biological signaling networks comprised of cellular components including signaling proteins and small molecule messengers control the many cell function in responses to various extracellular and intracellular signals including hormone and neurotransmitter inputs, and genetic events. Many signaling pathways have motifs familiar to electronics and control theory design. Feedback loops are among the most common of these. Using experimentally derived parameters, we modeled a positive feedback loop in signaling pathways used by growth factors to trigger cell proliferation. This feedback loop is bistable under physiological conditions, although the system can move to a monostable state as well. We find that bistability persists under a wide range of regulatory conditions, even when core enzymes in the feedback loop deviate from physiological values. We did not observe any other phenomena in the core feedback loop, but the addition of a delayed inhibitory feedback was able to generate oscillations under rather extreme parameter conditions. Such oscillations may not be of physiological relevance. We propose that the kinetic properties of this feedback loop have evolved to support bistability and flexibility in going between bistable and monostable modes, while simultaneously being very refractory to oscillatory states. (c) 2001 American Institute of Physics.     

Hysteretic behavior induced by an electroacoustic feedback loop in a thermo-acousto-electric generator

An active control method of the spatial distribution of the acoustic field is applied in a thermo-acousto-electric generator. An auxiliary acoustic source is used to force the self-sustained thermoacoustic oscillation in order to control the thermoacoustic amplification. The auxiliary source consists of a loudspeaker, located inside the loop-tube close to the main ambient heat exchanger, and supplied with a delayed signal through an electric feedback loop, comprising a phase-shifter and an amplifier, connected to a reference microphone. Experiments are performed on a prototype engine working with air at a static gauge pressure of 5 bars. Experimental results demonstrate how it is possible to tune the acoustic oscillations in order to increase the global performance of the generator, compared to the case without control, as well as the existence of a hysteretic behavior induced by the electroacoustic feedback loop itself, which leads to a discrepancy between the onset heat input and the offset one.     

Systematic error elimination by correction of variable interferometer aberrations

The possibility of reducing the variable systematic error in interferometry measurements via a higher degree of compensation of residual aberrations of the interferometer is shown theoretically and verified experimentally. It is proven that, in the case when interferometer aberrations are variable, one reference hologram is insufficient to correct aberrations. However, using several reference holograms on which the aberrations are recorded, it is possible to reconstruct interference patterns in which the influence of residual aberrations on the behavior of bands is reduced to a definite minimal level.     

Atmospheric-induced aberration correction using feedback interferometer

We present a practical adaptive optics system based on feedback interferometry. The system consists a polarizing radial-sheared Sagnac interferometer incorporating a multi-segmented mirror for wavefront correction. Correction of atmospheric aberration is demonstrated.     

Adaptive feedback cancellation in hearing aids with clipping in the feedback path

Adaptive linear filtering algorithms are commonly used to cancel feedback in hearing aids. The use of these algorithms is based on the assumption that the feedback path is linear, so nonlinearities in the feedback path may affect performance. This study investigated the effect on feedback canceller performance of clipping of the feedback signal arriving at the microphone, as well as the benefit of applying identical clipping to the cancellation signal so that the cancellation path modeled the nonlinearity of the feedback path. Feedback signal clipping limited the amount of added stable gain that the feedback canceller could provide, and caused misadjustment in response to high-level inputs, by biasing adaptive filter coefficients toward lower magnitudes. Cancellation signal clipping mitigated these negative effects, permitting higher amounts of added stable gain and less misadjustment in response to high-level inputs, but the benefit was reduced in the presence of the highest-level inputs.     

Tunable feedback loop for suppression of relaxation oscillations in a diode-pumped Nd: laser

An optoelectronic noise suppression circuit connected to the RF input of a commercial diode laser mount is presented. Adjustable phase shift and amplitude control enable efficient suppression of the relaxation oscillations of a diode laser-pumped Nd:YVO₄ laser at variable frequencies between a few hundred kHz and 1.8 MHz. At least 25 dB reduction of the relaxation oscillations at 500 kHz and 1.5 MHz noise peaks is demonstrated.     

Weight vector fluctuations in adaptive antenna arrays with nonlinear function in a feedback loop

We present the results of statistical analysis of adaptive antenna arrays with nonlinear function in the feedback loop, which are adjusted by an algorithm for recurrent inversion of a sample estimate of the input-signal correlation matrix with allowance for the weight-coefficient fluctuations. Using the perturbation-theory methods in the first (Born) approximation, we obtain expressions for the correlation function and power of the output signal of an adaptive antenna array and a formula for the covariance matrix of the weight-coefficient vector. The weight-coefficient fluctuations are shown to result in signal distortions at the antenna-array output and appearance of additional terms in the statistical characteristics of the antenna array. These terms are second-order infinitesimals with respect to adaptation coefficient. Fluctuations are first-order infinitesimals and, thus, are much greater in an antenna array without nonlinear function in the feedback loop. It is also shown that the weight-vector fluctuations are anisotropic, i.e., they are absent in the constraint direction, minimal in the interference direction, and maximal in all other directions. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 49, No. 2, pp. 164–172, February 2006.     

Evaluation and correction of aberrations in an optical correlator by phase-shifting interferometry

We propose a new method for evaluating and correcting aberrations in a Vander Lugt correlator. The technique is achieved with liquid-crystal displays of the correlator and allows the task to be performed in situ. We present the theory on which the method is based and the experimental results that we obtained by applying it in a convergent correlator.     

Spatial soliton-like structures in a thin-film system with a transverse photonic crystal in the feedback loop

The problem of existence and stability of soliton-like structures in a system consisting of a thin layer with resonance nonlinearity, a feedback mirror, and a photonic crystal located between them is studied. It is shown that, depending on the parameters of the photonic crystal, its presence can lead both to a change in the background on which localized structures are excited and to an increase in the interval of test pulse amplitudes necessary for the excitation of these structures.     

All-optical flip-flop based on an active Mach-Zehnder interferometer with a feedback loop

A novel architecture for an all-optical flip-flop is validated experimentally. The architecture comprises a single semiconductor optical amplifier-based Mach-Zehnder interferometer with an external feedback loop. The experimental results show optical bistable operation for a latching device with an on-off contrast ratio of 11 dB that employs set and reset pulses of less than 250 pJ, although the energy of these pulses could be greatly reduced by optical integration of the whole device.     

Charge qubit purification by an electronic feedback loop

We propose the manipulation of an isolated qubit by a simple instantaneous closed-loop feedback scheme in which a time-dependent electronic detector current is directly back-coupled into qubit parameters. As a specific detector model, we employ a capacitively coupled single-electron transistor. We demonstrate the stabilization of pure delocalized qubit states above a critical detector-qubit coupling. This electronic purification is independent of the initial qubit state and is accomplished after few electron jumps through the detector. Our simple scheme can be used for the efficient and robust initialization of solid-state qubits in quantum computational algorithms at arbitrary temperatures.     

Coherent combining of tunable erbium-doped fiber lasers by a single-mode fiber feedback loop

We experimentally demonstrate the coherent combining of two tunable erbium-doped fiber lasers by using a single-mode fiber feedback loop configuration. A single-mode fiber is arranged in the feedback loop to filter the far-field pattern, and the energy of desired in-phase mode is collected and injected into the resonators of two component fiber lasers. The coherently combined laser is tunable over a wide spectrum ranging from 1536 to 1569 nm, which means that the combining scheme is compatible with wavelength tuning. The effects and necessity of whether adopting polarization controlling measures or not in component lasers are investigated in detail. The results indicate that adding polarization controlling can improve the array＇s coherence, whereas it will decrease the output power and efficiency simultaneously.