High-speed directly modulated distributed feedback laser based on detuned loading and photon-photon resonance effect

A monolithic integrated two-section distributed feedback(TS-DFB)semiconductor laser for high-speed direct modulation is proposed and analyzed theoretically.The grating structure of the TS-DFB laser is designed by the reconstructionequivalent-chirp(REC)technique,which can reduce the manufacturing cost and difficulty,and achieve high wavelength controlling accuracy.The detuned loading effect and the photon-photon resonance(PPR)effect are utilized to enhance the modulation bandwidth of the TS-DFB laser,exceeding 37 GHz,while that of the conventional one-section DFB laser is only 16 GHz.When the bit rate of the non-return-to-zero(NRZ)signal reaches 55 Gb/s,a clear eye diagram with large opening can still be obtained.These results show that the proposed method can enhance the modulation bandwidth of DFB laser significantly.

Modulation bandwidth enhancement in monolithic integrated two-section DFB lasers based on the detuned loading effect

Modulation bandwidth enhancement in a directly modulated two-section distributed feedback(TS-DFB)laser based on a detuned loading effect is investigated and experimentally demonstrated.The results show that the 3-dB bandwidth of the TS-DFB laser is increased to 17.6 GHz and that chirp parameter can be reduced to 2.24.Compared to the absence of a detuned loading effect,there is a 4.6 GHz increase and a 2.45 reduction,respectively.After transmitting a 10 Gb/s non-return-to-zero(NRZ)signal through a 5-km fiber,the modulation eye diagram still achieves a large opening.Eight-channel laser arrays with precise wavelength spacing are fabricated.Each TS-DFB laser in the array has side mode suppression ratios(SMSR)>49.093 dB and the maximum wavelength residual<0.316 nm.

Enhanced optical molasses cooling for Cs atoms with largely detuned cooling lasers

We report a detailed study of the enhanced optical molasses cooling of Cs atoms,whose large hyperfine structure allows to use the largely red-detuned cooling lasers.We find that the combination of a large frequency detuning of about-110 MHz for the cooling laser and a suitable control for the powers of the cooling and repumping lasers allows to reach a cold temperature of^5.5μK.We obtain 5.1×10^7 atoms with the number density around 1×10^12 cm^-3.Our result gains a lower temperature than that got in other experiments,in which the cold Cs atoms with the temperature of^10μK have been achieved by the optical molasses cooling.

Localization of elastic waves in one-dimensional detuned phononic crystals with flexoelectric effect

Although forbidden band effect in perfectly periodic phononic crystals(PC)is very attractive,random disordered(i.e.detuning)phenomenon is inevitable in engineering processing,thus explor-ing the effect of detuning on the wave characteristics of PC becomes a necessity.In this study,fundamental governing equa-tions and boundary conditions are derived from the principle of virtual work.Wave characteristics and localization factor of one-dimensional(1D)detuned nano-PC are investigated based on the transfer matrix method,with flexoelectric effect duly accounted for.Subsequently,with BaTiO_(3)􀀀SrTiO_(3) nano-PC taken for illustration,forbidden band properties and localization factor of 1D elastic waves in harmonic and detuned states are systematically character-ized.It is demonstrated that localization factor can characterize the energy band structure of 1D PC perfectly.Flexoelectric effect tends to widen the width of forbidden band with increasing detuning,and detuning is linearly related to the bandwidth.The forbidden band is more sensitive to flexoelectric coefficient detuning than thickness detuning.The research results provide useful theoretical guidance for designing high-frequency nanoscaled devices with the function of filtering based on band gap effect of nano-PC.

Preparation of entangledW states based on the cavity QED system

We present a qubit-loss-free(QLF)fusion scheme for generating large-scale atom W states in cavity quantum electrodynamics(QED)system.Compared to the most current fusion schemes which are conditioned on the case where one particle can be extracted from each initial W state to the fusion process,our scheme will access one or two particles from each W state.Based on the atom–cavity-field detuned interaction,three jWin+m+t states can be generated from the jWin,jWim,and jWit states with the help of two auxiliary atoms,and three jWin+m+t+q states can be generated from jWin,jWim,jWit,and a jWiq state with the help of three auxiliary atoms.Comparing the numerical simulations of the resource cost of fusing three small-size W states based on the previous schemes,our fusion scheme seems to be more efficient.This QLF fusion scheme can be generalized to the case of fusing k different or identical particle W states.Furthermore,with no qubit loss,it greatly reduces the number of fusion steps and prepares W states with larger particle numbers.

Teleportation of Unknown Atomic Entangled States Using GHZ Class States

We propose two physical schemes, which can teleport unknown atomic entangled states from user A （Alice） to user B （Bob） via GHZ class states as quantum channel The two schemes are both based on cavity QED techniques. In the two schemes, teleportation and distillation procedures can be realized simultaneously. The second teleportation scheme is more advantageous than the first one.

Quantum dynamic behaviour in a coupled cavities system

The dynamic behaviour of the two-site coupled cavities model which is doped with ta wo-level system is investi-gated. The exact dynamic solutions in the general condition are obtained via Laplace transform. The simple analytical solutions are obtained in several particular cases, which demonstrate the clear and simple physical picture for the quan-tum state transition of the system. In the large detuning or hoppling case, the quantum states transferring between qubits follow a slow periodic oscillation induced by the very weak excitation of the cavity mode. In the large coupling case, the system can be interpreted as two Jaynes-Cummings model subsystems which interact through photon hop between the two cavities. In the case of λ≈△〉〉 g, the quantum states transition of qubits is accompanied by the excitation of the cavity, and the cavity modes have the same dynamic behaviours and the amplitude of probability is equM to 0.25 which does not change with the variation of parameter.

The influences of dipole-dipole interaction and detuning on the sudden death of entanglement between two atoms in the Tavis-Cummings model

The influences of dipole-dipole interaction and detuning on the entanglement between two atoms with different initial tripartite entangled W-like states in the Tavis Cummings model have been investigated by means of Wootters＇ concurrence, respectively. The results show that the entanglement between the two atoms can be enhanced via appropriately tuning the strength of dipole-dipole interaction of two atoms or the detunings between atom and cavity, and the so-called sudden death effect can he weakened simultaneously.

Spectrum of the Micromaser with Kerr Medium

We have established the master equation for the micromaser with Kerr medium field density operator, studied the spectrum of the micromaser with Kerr medium and analyzed the influence of Kerr effect and the detuning on the spectrum. In the thermal-atom regime, we find that Kerr effect broadens linewidth D and increases frequency-shift S, and that the detuning Δ narrows linewidth D and increases frequency-shift S as a whole. Moreover Kerr effect leads to oscillatings more rapidly in the resonance peaks, which means that it causes quantum noise. As a whole, with the increase of cavity-length L, the linewidth D and frequency-shift S gradually increase.

Stable single-mode operation of 894.6 nm VCSEL at high temperatures for Cs atomic sensing

In this paper, stable single-mode operation at high temperatures is produced by the surface-relief-integrated vertical cavity surface emitting laser（VCSEL）. The gain-cavity mode detuning technique is employed to realize high operating temperatures for the VCSEL. The surface relief is etched in the centre of the top side as a mode discriminator for the fundamental mode output, and the threshold current minimum is 1.94 mA at high temperatures by the gain-cavity mode detuning technique. Maximum single-fundamental-mode output power of 0.45 mW at 80℃ is obtained, and the side mode suppression ratios（SMSRs） are more than 30 dB with increasing temperature and current, respectively.

Optical bistability and multistability in a three-level △-type atomic system under the nonresonant condition

Under a nonresonant condition, we theoretically investigate hybrid absorptive-dispersive optical bistability and multistability behaviours in a three-level △-type system by using a microwave field to drive a hyperfine transition between two upper excited states inside a unidirectional ring cavity. We find that the optical bistability and multistability behaviours can be controlled by adjusting the intensity of the microwave field or the intensity of the coherent coupling field. Furthermore, our studies show an interesting phenomenon of the transition from the optical bistability to the optical multistability only by changing the negative detuning of the coupling field into the positive detuning of the coupling field.

Frequency sensitivity of the passive third harmonic superconducting cavity for SSRF

A 1.5 GHz passive third harmonic superconducting cavity was proposed to improve the beam quality and lifetime in the Shanghai Synchrotron Radiation Facility Phase-II beamline project.Lifetime improvement highly depends on the resonant frequency of the passive third harmonic superconducting cavity.It is important that the operating frequency of the cavity is within the design range and the cavity has reasonable mechanical stability.A simulation method for the multiphysics coupled analysis has been developed based on the ANSYS code.Multiphysics coupled simulations have been performed under different conditions,such as etching,evacuation,cooling,and preloading.Analyses of mechanical modes and structural stress have been executed.A possible stiffening ring method for the two-cell superconducting niobium cavity has been investigated.In this paper,we present a multiphysics coupled analysis of the third harmonic cavity using a finite element analysis code.The results of the analysis show that a reliable frequency for the cavity after electron beam welding is 1498.033 MHz,and the corresponding frequency of the pre-tuning goal is 1496.163 MHz.A naked cavity is a reasonable option based on structural stress and mechanical modal analyses.A frequency range of±500 kHz and limiting tolerable displacement of±0.35 mm are proposed for the design of the frequency tuner.

A non-invasive diagnostic method of cavity detuning based on a convolutional neural network

As modern accelerator technologies advance toward more compact sizes,conventional invasive diagnostic methods of cavity detuning introduce negligible interference in measurements and run the risk of harming structural surfaces.To overcome these difficulties,this study developed a non-invasive diagnostic method using knowledge of scattering parameters with a convolutional neural network and the interior point method.Meticulous construction and training of the neural network led to remarkable results on three typical acceleration structures:a 13-cell S-band standing-wave linac,a 12-cell X-band traveling-wave linac,and a 3-cell X-band RF gun.The trained networks significantly reduced the burden of the tuning process,freed researchers from tedious tuning tasks,and provided a new perspective for the tuning of side-coupling,semi-enclosed,and total-enclosed structures.

High-precision high-voltage detuning system for HIAF-SRing electron target

The development of a detuning system for the precision control of electron energy is a major challenge when electron targets are used in ion-storage rings.Thus,a high-precision,high-voltage,detuning system was developed for the electron target of a high-intensity heavy-ion accelerator facility-spectrometer ring(HIAF-SRing)to produce accurate electron-ion relative energies during experiments.The system consists of auxiliary,and high-voltage detuning power supplies.The front stage of the auxiliary power supply adopts an LCC resonant converter operating in the soft-switching state and an LC filter for a sinusoidal waveform output in the post-stage.The detuning power supply is a high-voltage pulse amplifier(HVPA)connected with a high-voltage DC(HVDC)module in series.In this paper,the design and development of the detuning system are described in detail,and the test bench is presented.The test results demonstrated that the detuning system conforms to the technical specifications of the dielectronic recombination(DR)experiment.Finally,a Fe15+DR spectrum was measured using the detuning system.The experimental data demonstrated a good experimental resolution and verified the reliability and feasibility of the design.

Synchronously Pumped Mode-Locked 1.89 μm Tm-Doped Fiber Laser with High Detuning Toleration

We propose and demonstrate a synchronously pumped mode-locked Tm-doped fiber（TDF） laser without any extra mode-locking elements. Pumped by a 1.56 μm pulse fiber laser, the TDF laser generates 1.17 ps pulses with a spectral width of 9.7 nm and a repetition rate of 9.33 MHz. The emission wavelength is tunable along with the cavity length detuning in a wide range of 3 mm. The high detuning toleration is beneficial to achieve high temperature and vibration stability in all-fiber configuration lasers.

Mechanical analysis of low β 162.5 MHz taper HWR cavity

Peking University is designing a superconducting taper-type half-wave resonator （HWR） withfrequency of 162.5 MHz and β of 0.09 for high current proton beam （about 100 mA） and deuteron beam （about50 mA） acceleration. For this kind of SRF cavity, mechanical analysis is important to predict the frequencyshift caused by the cavity deformation. ANSYS codes are used to analyse the frequency shift caused by heliumbath pressure and Lorentz force, and the tuning range. Simulation results show that this HWR cavity has pressure sensitivity as low as 31.1 Hz/kPa and Lorentz force detuning coefficient of --0.41 Hz/（MV · m ^-1）e. Thetuning range of the cavity is ±4 177 kHz which is wide enough for compensating the possible frequency shift.The mechanical property of the cavity permits the cavity＇s stable operation.

Spectrum of the Micromaser with Kerr Medium

We have established the master equation for the micromaser with Kerr medium field density operator,studied the spectrum of the micromaser with Kerr medium and analyzed the influence of Kerr effect and the detuning onthe spectrum. In the thermal-atom regime, we find that Kerr effect broadens linewidth D and increases frequency-shiftS, and that the detuning A narrows linewidth D and increases frequency-shift S as a whole. Moreover Kerr effect leadsto oscillatings more rapidly in the resonance peaks, which means that it causes quantum noise. As a whole, with theincrease of cavity-length L, the linewidth D and frequency-shift S gradually increase.

Sudden death of entanglement of two atoms interacting with thermal fields

We investigate the entanglement dynamics of two initially entangled atoms each interacting with a thermal field. We show that the two entangled atoms become completely disentangled in a finite time and that the lost information cannot return to the atomic system when the mean photon number of the thermal field exceeds a critical value （3.3584）, even though the whole system is lossless. Then we study how the detuning between the atomic transition frequency and the field frequency and the disparity between two coupling rates would affect the evolution of the entanglement of the atomic system.

Entanglement sudden death of two atoms interacting with a cavity via the two-photon process in a strong-driving-assisted system

We examine the entanglement dynamics between two strongly driven atoms off-resonantly coupled with a singlemode cavity via the two-photon process with the help of negativity in two different types of initial states. The results show that entanglement sudden death may occur under both the above conditions and the sudden death effect can be monitored by modulating the atom-cavity detunings. Furthermore, we also find an atomic decoherence-free subspace so that the initial entanglement between two atoms remains invariable in application.

Generation of steady four-atom decoherence-free states via quantum-jump-based feedback

A scheme is presented tor generating steady tour-atom decoherence-tree states via tour atoms with the Raman level configuration interacting with a single-mode vacuum cavity field by using quantum-jump-based feedback. The scheme meets the condition of a strongly dissipative cavity easily and has a simplified feedback control. Although the spontaneous emission still plays a negative role in the proposed system, we can improve the feedback control to reduce its effect.