Resonant Feedback Research Articles

Frequency stability of an RF oscillator with an MEMS-based encapsulated resonator**Project supported by the National Natural Science Foundation of China (Nos. 61234007, 61404136) and the State Key Development Program for Basic Research of China (Nos. 2011CB933102, 2013YQ16055103).

This paper presents a high-Q RF MEMS oscillator consisting of a micro-disk resonator and low noise feedback circuits. The oscillator has high frequency stability and low phase noise. The two-port resonator was hermetically encapsulated using low-cost Sn-rich Au–Sn solder bonding, which significantly improves the frequency stability. A low noise oscillator circuit was designed with a two-stage amplifying architecture which effectively improves both the frequency stability and phase noise performance. The measured phase noise is −96 dBc/Hz at 1 kHz offset and −128 dBc/Hz at far-from-carrier offsets. Moreover, the medium-term frequency stability and Allan deviation of the oscillator are ±4 ppm and 10 ppb, respectively. The oscillator is a promising component in future wireless communication application.

Independent Operation of DFIG-Based WECS Using Resonant Feedback Compensators Under Unbalanced Grid Voltage Conditions

This paper presents an independent operation of the rotor-side converter (RSC) and grid-side converter (GSC) for a doubly fed induction generator (DFIG)-based wind energy conversion system under unbalanced grid voltage conditions. In this paper, the RSC is controlled to achieve four different control targets, including balanced stator current, sinusoidal rotor current, smooth stator active and reactive powers, and constant DFIG electromagnetic torque. The GSC is commanded to keep the dc voltage at a constant value. Additional feedback compensators using resonant regulators for the RSC are employed, and the decompositions of the positive and negative sequence components and calculations of the rotor negative current references can be avoided. Another similar compensator is used in the GSC to suppress the dc voltage fluctuates and remove the GSC reactive power oscillations without the stator or rotor power information. The proposed method can make the RSC and GSC available to an independent operation with a simple implementation for higher reliability. The experimental results demonstrate the effectiveness of the proposed control strategy for both the RSC and GSC under unbalanced grid voltage conditions.

Low intensity noise and narrow line-width diode laser light at 540 nm

We present a convenient method to generate high quality single-frequency green light at a wavelength of 540 nm. It consists of a noise suppressed external cavity diode laser at a wavelength of 1080 nm by optical filtering and resonant optical feedback, and a frequency doubling of the fundamental light with an a-cut KTP crystal. Highly efficient conversion is realized by type II non-critical phase matching. A stable single-frequency operation with a maximum power of about 20 mW is performed for more than 3 h. Both the intensity noise and line-width reach the level of a monolithic nonplanar ring laser, which is well known for its extraordinarily narrow line-width and extremely low noise among available single-frequency operating lasers.

Ultra-narrow linewidth DFB-laser with optical feedback from a monolithic confocal Fabry-Perot cavity.

We present a compact, ultra-narrow-linewidth semiconductor laser based on a 780 nm distributed feedback diode laser optically self-locked to a mode of an external monolithic confocal Fabry-Perot resonator. We characterize spectral properties of the laser by measuring its frequency noise power spectral density. The white frequency noise levels at 5 Hz(2)/Hz above a Fourier frequency as small as 20 kHz. This noise level is more than five orders of magnitude smaller than the noise level of the same solitary diode laser without resonant optical feedback, and it is three orders of magnitude smaller than the noise level of a narrow linewidth, grating-based, extended-cavity diode laser. The corresponding Lorentzian linewidth of the laser with resonant optical feedback is 15.7 Hz at an output power exceeding 50 mW.

Optimal and resonant time-delayed feedback control of unstable steady states: self-adaptive tuning of coupling phase

We suggest an optimized scheme for the time-delayed feedback control of an unstable fixed point. For the normal form of an unstable focus, we derive the optimal feedback phase that yields the strongest stability and the shortest transient times for control. For this optimal value, the domain of control becomes largest in the plane of feedback strength and time delay. We present an adaptive algorithm for an automatic adjustment of the feedback phase for the optimal control without knowledge for the intrinsic parameters of the system. We extend the optimized control scheme to a network of supercritical Hopf normal forms coupled with delay. Analysis of eigenvalues using a master stability function approach shows that amplitude death is considerably facilitated if the coupling phase is equal to the optimized coupling phase.

Narrow linewidth low frequency noise Er-doped fiber ring laser based on femtosecond laser induced random feedback

We propose and demonstrate a narrow linewidth, low frequency noise Er-doped fiber ring laser with resonant feedback in a femtosecond laser induced random medium of deep refractive index modulation in three dimensions. Eight concatenated single-mode fiber segments about 1 cm long, each carry a total of 8 × 500 randomly spaced laser-written-planes. Numerous low-finesse spectral filters are formed to significantly suppress sub-cavity modes, ensuring single-mode operation within a wavelength-locking range. The linewidth of the laser is 2.1 kHz with 58 dB side-mode-suppression-ratio. The frequency noise is ∼1 Hz/Hz1/2 above 1 kHz, and the frequency jitter is ∼1.8 × 10−12 over 100 s.

3 kHz PIV/OH-PLIF measurements in a gas turbine combustor at elevated pressure

This study was designed to test the feasibility of acquiring simultaneous PIV/OH-PLIF measurements at multi-kHz rates in a turbulent swirl flame at pressures relevant to modern industrial gas turbine combustors. To accomplish this, particle image velocimetry (PIV) and planar laser-induced fluorescence of the hydroxyl radical (OH-PLIF) were applied simultaneously at 3kHz to study the dynamics of a lean partially-premixed turbulent swirl-stabilized flame of natural gas in an optically accessible, high-pressure combustion test rig at 5bars. With 0.25mJ/pulse at 283nm for the OH-PLIF measurements, an average signal-to-noise ratio (SNR) of 4.1 was achieved over a region measuring 20×80mm. Absorption of the excitation laser proved to be the greatest challenge in this study, resulting in a significant variation in SNR from one side of the OH-PLIF images to the other. A procedure based on modeling the absorption according to the mean OH-distribution was used to semi-quantitatively correct for this effect. A gradient-based edge-detection algorithm was used to identify reaction zone locations in the resulting images. These were used to compute mean distributions of the flame surface density.With 2.5mJ/pulse at 532nm for the PIV system, velocity fields measuring 20×80mm were measured at a resolution of 1.25mm. Consistent with prior measurements in the burner, the flame shows strong thermo-acoustic pulsation, with a peak frequency of 388Hz. Phase-averages of the PIV and OH∗ data indicate these pulsations are driven by the same resonant feedback mechanism responsible for thermo-acoustic pulsation in the burner at atmospheric pressure. No evidence of a precessing vortex core, known to dominate the flow-field of the burner at atmospheric-pressure conditions, was observed.

Chaotic Oscillations in a CMOS Inverter Coupled With ESD Protection Circuits Under Radio Wave Excitation

This study demonstrates the presence of chaotic oscillations in standard CMOS circuits. At radio-frequencies, ordinary digital circuits can show unexpected nonlinear responses. We examine a CMOS inverter coupled with electrostatic discharging (ESD) protection circuits, designed with 0.35 $\mu$ m CMOS technology, for evidence of its chaotic oscillations. As the circuit is directly driven by a radio-frequency signal, the circuit enters a chaotic dynamic regime when the input frequency is higher than the maximum operating frequency of CMOS inverter. We observe an aperiodic signal, a broadband spectrum, and a complex spectrum. We discuss the nonlinear physical effects in the given circuit: ESD diode rectification, dc bias shift due to a nonquasi-static regime operation of the ESD PN-junction diode, and a nonlinear resonant feedback current path. In order to predict these chaotic dynamics, we develop a transistor-based model, and compare it with the experimental results. To verify the presence of chaotic oscillations mathematically, we develop an ordinary differential equation model with the circuit-related nonlinearities. The largest Lyapunov exponents are calculated to verify the chaotic oscillations. The importance of this study derives from investigating chaotic oscillations in standard CMOS circuits as circuit-effects due to high-intensity electromagnetic signals.

A directional, ultrafast and integrated few-photon source utilizing the interaction of electron beams and plasmonic nanoantennas

The possibility of using plasmonic nanoantennas in interaction with electron beams to form a wideband few-photon source is demonstrated here using numerical investigations. The generated photons are guided inside a waveguide, which is coupled to the nanoantennas. Using two replicas of a nanoantenna at an optimized distance, and providing a guided path for the emitted photon from the first nanoantenna to the second one by positioning a resonator feedback element, it is possible to form an interference between the photons generated by the two nanoantennas. The interference phenomenon is used to produce a unidirectional flow of power in the waveguide, as well as a high collection efficiency. The investigations are carried out in a self-consistent way, utilizing a conjugate Maxwell and Lorentz system of equations, in order to be able to simulate the modulation of the electron velocity due to the interaction with an optical system which is initially at rest.

On Flow-Acoustic Resonant Interactions in Transitional Airfoils

We discuss our recent experimental and preliminary numerical efforts examining resonant feedback mechanism of flow-acoustic interactions in airfoil transitional boundary layers. Experimentally recorded unsteady responses of loaded, transitional NACA0012 airfoil with tripped suction or pressure sides confirm the presence of shifted ladder-type tonal structures with dual velocity dependence in the acoustic signal previously reported in the literature. The installation of upstream grid generating low-intensity turbulence appears to eliminate the feedback mechanism leaving just a single velocity dependence for the dominant spectral components. Complementary numerical efforts employ a high-order Navier-Stokes solver implementing low-pass filtering of poorly resolved high-frequency solution content to retain numerical accuracy and stability over the range of transitional flow regimes. Within the scope of 2D analysis, the conducted numerical experiments particularly investigate the behavior of the boundary-layer statistical moments during the transitional flow regimes characterized by the presence of the acoustic feedback, and address sensitivity of the latter to the flow Reynolds number.

Phase noise and jitter reduction by optical feedback on passively mode-locked quantum-dot lasers

The noise properties of the pulse trains of passively mode-locked 40 GHz quantum-dot lasers subject to optical feedback are investigated in detail. Five different feedback regimes are discovered and the clearly identified regime of resonant optical feedback is further examined. The feedback parameters yielding minimum phase noise are determined. Here, the radio-frequency (RF)-line-width is reduced from its original value by 99% to 1.9 kHz. The corresponding pulse-to pulse jitter of 23 fs is a record low value for passively mode-locked 40 GHz quantum-dot lasers.

Active vibration control using delayed resonant feedback

Delayed feedback (DF) control is a well-established technique to suppress single frequency vibration of a non-minimum phase system. Modal control is also a well-established technique to control multiple vibration modes of a minimum phase system. In this paper these techniques are combined to simultaneously suppress multiple vibration modes of a non-minimum phase system involving a small time delay. The control approach is called delayed resonant feedback (DRF) where each modal controller consists of a modal filter to extract the target mode signal from the vibration response, and a phase compensator to account for the phase delay of the mode. The methodology is first discussed using a single mode system. A multi-mode system is then studied and experimental results are presented to demonstrate the efficacy of the control approach for two modes of a beam. It is shown that the system behaves as if each mode under control has a dynamic vibration absorber attached to it, even though the actuator and the sensor are not collocated and there is a time delay in the control system.

Competition of modes and self-modulation instability in dynamics of coherent friction: A review

A structural pattern of the autowave transfer of the pulse and energy in dry and boundary friction systems is proposed. The coherence of friction is provided by a nonlinear excitation wave field with a nucleus-shell structure. A matching nucleus is concentrated in the friction contact and the physical volume of the friction counterbodies plays a role of a feedback resonator. The envelope velocity of a wave packet is a key parameter of the matching of movement in the autowave mode. Physical mechanisms and specific examples of the competition of modes in dynamic tribosystems that lead to the self-modulation instability of these modes are considered. A comparative analysis of the coherent transfer of the pulse and energy in external and internal friction systems is carried out to describe nonstationary modes and turbulence.

Nanoimprinted polymer lasers with threshold below 100 W/cm^2 using mixed-order distributed feedback resonators

Organic semiconductor lasers were fabricated by UV-nanoimprint lithography with thresholds as low as 57 W/cm(2) under 4 ns pulsed operation. The nanoimprinted lasers employed mixed-order distributed feedback resonators, with second-order gratings surrounded by first-order gratings, combined with a light-emitting conjugated polymer. They were pumped by InGaN LEDs to produce green-emitting lasers, with thresholds of 208 W/cm(2) (102 nJ/pulse). These hybrid lasers incorporate a scalable UV-nanoimprint lithography process, compatible with high-performance LEDs, therefore we have demonstrated a coherent, compact, low-cost light source.

Nanoimprinted Organic Semiconductor Laser Pumped by a Light‐Emitting Diode

An organic semiconductor laser, simply fabricated by UV-nanoimprint lithography (UV-NIL), that is pumped with a pulsed InGaN LED is demonstrated. Molecular weight optimization of the polymer gain medium on a nanoimprinted polymer distributed feedback resonator enables the lowest reported UV-NIL laser threshold density of 770 W cm(-2) , establishing the potential for scalable organic laser fabrication compatible with mass-produced LEDs.

Organic single-crystal light-emitting transistor coupling with optical feedback resonators.

Organic light-emitting transistors (OLETs) are of great research interest because they combine the advantage of the active channel of a transistor that can control the luminescence of an in-situ light-emitting diode in the same device. Here we report a novel single-crystal OLET (SCLET) that is coupled with single crystal optical feedback resonators. The combination of single-crystal waveguides with native Fabry-Perot cavities, formed by parallel crystal edges, drastically lowers the threshold energy for spectral narrowing and non-linear intensity enhancement. We apply this structure to SCLETs and demonstrate the first fabrication of a SCLET with the optical feedback resonators.

Phase and intensity dynamics of a two-frequency laser submitted to resonant frequency-shifted feedback

We propose an experimental and theoretical study of the dynamical regimes observed when the two modes of a two-frequency solid-state laser are coupled by frequency-shifted optical feedback. The detuning associated to the feedback is close to the frequency of the laser relaxation oscillations. Special attention is devoted to the dynamics of the phase of the beat note between the modes relative to the phase of an external radio-frequency reference. In particular, we analyze in detail the transition from phase locking to the external reference to phase drift. This transition occurs through a window of frequency locking without phase locking [Th\'evenin, Romanelli, Vallet, Brunel, and Erneux, Phys. Rev. Lett. 107, 104101 (2011)]. Furthermore, a large variety of dynamical behaviors, such as weak intensity modulation, self-pulsing, and chaos, are experimentally found. These results are successfully reproduced by a theoretical model based on coupled rate equations. The extension of the model to the description of other active media such as semiconductor lasers is briefly outlined.

Random fiber laser of POSS solution-filled hollow optical fiber by end pumping

Random fiber laser is obtained by end pumping a hollow optical fiber (HOF) filled with a dispersive solution of polyhedral oligomeric silsesquioxanes (POSS) nanoparticles and laser dye pyrromethene 597 (PM597) in carbon disulfide (CS2), in which the concentration is 1.5×10−2M for PM597 and 18.5wt% for POSS, respectively. It is found that the pump light at the one end of the liquid core optical fiber (LCOF) can pass the whole length of LCOF because the POSS nanoparticles were dispersed in CS2 at a molecular level (1–3nm) with high stability and without sedimentation. Above the threshold pump energy (∼0.81mJ) the random fiber laser appears coherent and resonant feedback multimode lasing in the weakly scattering system. For the LCOF containing PM597 with the same concentration and no POSS nanoparticles, there occurs only ASE that can be observed under the same experimental condition.

A nanoimprinted, optically tuneable organic laser

We present a means to optically tune the emission of an organic laser reversibly over a range of 14 nm. This is enabled by incorporating a photochromic spiropyran (10,30-dihydro-10,30,30-trimethyl-6-nitrospiro[2H-1-benzopyran-2,20-(2H)-indole]) into the gain medium formed by the laser dye Rhodamine 700 in poly(methyl methacrylate). This is structured by nanoimprinting to form a distributed feedback resonator. Exposure of the device to ultraviolet and visible light leads to a reversible refractive index change of the photochromic additive and the gain medium, which enables the continuous shift of the laser wavelength. This is possible without compromising the optical gain properties of the device.

Efficient phase noise suppression of an external-cavity diode-laser by optical filtering and resonant optical feedback

We experimentally demonstrated that the excess phase noise of an external cavity diode laser (ECDL) can be efficiently suppressed by optical filtering and resonant optical feedback techniques. A suppression of more than 40 dB phase noise was obtained for the first time using these methods. It made that the phase noise of the ECDL reached shot noise level from 15 MHz. This opens a new way to reduce the phase noise of diode lasers.