Phase control of light amplification in steady and transient processes in an inverted-Y atomic system with spontaneously generated coherence

We investigate the effects of spontaneously generated coherence （SGC） on both the steady and transient gain properties in a four-level inverted-Y-type atomic system in the presence of a weak probe, two strong coherent fields, and an incoherent pump. For the steady process, we find that the inversionless gain mainly origins from SGC. In particular, we can modulate the inversionless gain by changing the relative phase between the two fields. Moreover, the amplitude of the gain peak can be enhanced and the additional gain peak can appear by changing the detuning of the coupling field. As for the transient process, the transient gain properties can also be dramatically affected by the SGC. Compared to the case without SGC, the transient gain can be greatly enhanced with completely eliminated transient absorption by choosing the proper relative phase between the two fields. And the inverted-Y-type system with SGC can be simulated in both atomic and semiconductor quantum well systems avoiding the conditions of SGC.     

Quantum interference effects on the probe amplification without and with inversion in a four-level system

A four-level system driven by two coherent fields is considered. It is shown that in the presence of an incoherent pump, the probe gain at a short wavelength can be achieved due to the quantum interference. Our density matrix calculation provides the conditions for probe amplification from different origins, including gain without population inversion on any state basis, gain with population inversion on the dressed-state basis, and gain with population inversion on the bare-state basis. Also, by controlling the Rabi frequency of the coupling field a total change from non-inversion to inversion can be achieved which does not depend on the intensity of the incoherent pump.     

Probe gain via four-wave mixing based on spontaneously generated coherence

We have studied the probe gain via a double-Λ atomic system with a pair of closely lying lower levels in the presence of two probe and two coherent pump fields. The inversionless gain can be realized by using nondegenerate four-wave mixing under the condition of spontaneously generated coherence(SGC) owing to near-degenerate lower levels. Note that by using SGC, two probe fields can be amplified with more remarkable amplitudes, and the gain spectra of an extremely narrow linewidth can be obtained. Last but not least, our results show that the probe gain is quite sensitive to relative phases due to the SGC presence which allows one to modulate the gain spectra periodically by phase modulation, and can also be influenced by all laser field intensities and frequencies, and the angles between dipole elements.     

Amplification mechanism and effects of spontaneously generated coherence on the probe gain in a four-level system

We investigate the amplification mechanism in a four-level system by both the density-matrix and quantum-jump approaches. We show that the asymmetry between three-photon stimulated emission and one-photon absorption process is responsible for the inversionless amplification of the probe field. We also investigate the effects of spontaneously generated coherence (SGC) on the probe gain, and find that due to the SGC, the probe gain can be greatly enhanced and can be modulated by changing the relative phase between the applied fields.     

Design of Raman-parametric fiber amplifier for wavelength division multiplex transmission system

We optimize the novel configuration of a hybrid fiber amplifier - Raman assisted-fiber-based optical parametric amplifier （R-FOPA）, in which the parametric gain and Raman gain profiles are combined to achieve a flat composite gain profile. The pump powers and the fiber length in the hybrid amplifier are effectively optimized by genetic algorithm （GA） scheme. The optimization results indicate that the RFOPA can achieve a 200-nm flat bandwidth spectrum with the gain of 20 dB and ripple of less than 4 dB.     

Effects of relative phase on transient evolution in an open resonant ladder-type atomic system

In an open ladder-type resonant atomic system, variation in relative phase between probe and driving fields does not affect the transient evolution of populations, but it has remarkable effects on gain and dispersion of the probe field. No matter whether an incoherent pump is present or absent, transient and stationary gains without inversion (GWI) always can be obtained by choosing an appropriate value of the relative phase. When the incoherent pump is absent,the values of transient and stationary GWIs are much larger and the time interval required to reach the stationary value is longer than those when the incoherent pump is present. Varying the exit rate and the ratio between injection rates can obviously change the phase-dependent GWI. In addition, in the transient evolution process, the phenomenon of high dispersion (refractive index) without absorption occurs at some values of relative phase. In the corresponding closed system, the stationary GWI can be obtained by choosing an appropriate value of relative phase only when incoherent pump exists, moreover the gain is smaller than that in the open system.     

Design of Raman-parametric fiber amplifier for wavelength division multiplex transmission system

We optimize the novel configuration of a hybrid fiber amplifier-Raman assisted-fiber-based optical parametric amplifier (R-FOPA),in which the parametric gain and Raman gain profiles are combined to achieve a flat composite gain profile.The pump powers and the fiber length in the hybrid amplifier are effectively optimized by genetic algorithm (GA) scheme.The optimization results indicate that the R- FOPA can achieve a 200-nm flat bandwidth spectrum with the gain of 20 dB and ripple of less than 4 dB.     

Modelling of passively Q-switched lasers with intracavity Raman conversion

We present a model of passively Q-switched Raman lasers by utilizing the rate equations. The intracavity fun-damental photon density, Raman photon density and the initial population-inversion density of the gain medium are assumed to be of Gaussian spatial distributions. These rate equations are normalized by introducing some synthetic parameters and solved numerically, and a group of general curves are generated. Prom these curves we can understand the dependence of the Raman laser pulse characteristics on the parameters about the pumping, the gain medium, the Raman medium and the resonator. An illustrative calculation for a passively Q-switched Nd^3＋：GdVO4 self-Raman laser is presented to demonstrate the usage of the curves and related formulas.[第一段]     

Effect of spontaneously generated coherence on inversionless lasing gain in an atomic system with Doppler broadening

We have studied the effect of the spontaneously generated coherence (SGC) on gain of lasing without inversion (LWI) in a closed three-level $\Lambda $-type atomic system with Doppler broadening. It is shown that, regardless of the driving and probe fields being co- or counter-propagating, at a suitable value of the Doppler width, we can obtain a much larger LWI gain with SGC than that without SGC; and the region of the LWI gain spectrum with SGC is obviously larger than that without SGC. When the Doppler width takes a constant value, the gain does not monotonically decrease or increase with increasing strength of SGC, the largest LWI gain can be obtained by adjusting strength of SGC. Generally speaking, the co-propagating probe and driving fields is favourable to obtain a larger LWI gain.     

Dissipative quantum phase transition in a biased Tavis-Cummings model

We study the dissipative quantum phase transition(QPT)in a biased Tavis–Cummings model consisting of an ensemble of two-level systems(TLSs)interacting with a cavity mode,where the TLSs are pumped by a drive field.In our proposal,we use a dissipative TLS ensemble and an active cavity with effective gain.In the weak drive-field limit,the QPT can occur under the combined actions of the loss and gain of the system.Owing to the active cavity,the QPT behavior can be much differentiated even for a finite strength of the drive field on the TLS ensemble.Also,we propose to implement our scheme based on the dissipative nitrogen-vacancy(NV)centers coupled to an active optical cavity made from the gainmedium-doped silica.Furthermore,we show that the QPT can be measured by probing the transmission spectrum of the cavity embedding the ensemble of the NV centers.     

Incoherent pump influence on the transient gain in a V-type system with spontaneously generated coherence

We study the influence of the incoherent pump field on the transient response of a three-level V-type system with two near-degenerate excited states. In this system due to interaction of the field with the vacuum an interference term appears, the spontaneously generated coherence term. We find that due to the incoherent pump the transient proprieties of the atomic system can be significantly modified. The enhancement of the transient gain can be obtained for proper values of the rate of the incoherent pump field and relative phase between the two fields.     

非相干泵浦对原子系统位相相关吸收性质的影响

研究发现在具有真空诱导相干性的开放和封闭ladder 型原子系统中,探测场和驱动场之间相对位相对吸收性质有重要的调制作用,同时非相干泵浦对位相相关的吸收性质有显著的影响.在这两种系统中,通过调节非相干泵浦,可以实现从吸收到增益和从粒子数反转激光到无粒子数反转激光的转化,并可获得最大的无反转激光增益.但是,当非相干泵浦不存在时,对相对位相的任意值,在封闭系统中只出现吸收而不能得到无反转激光增益;然而在开放系统中,对于适当的相对位相值甚至可以产生比有非相干泵浦时更大的无反转激光增益.     

Impact of Interacting Quantum Coherence via Decays and Incoherent Pumping on Transient and Steady-State Behaviors of Absorption

The effects from quantum coherence via decays and incoherent pumping on the lasing without inversion(LWI) are investigated in a four-level system.By analyzing the transient and steady-state behaviors of the absorption,we find that the gain occurs when both spontaneously generated coherence(SGC) and coherence induced by incoherent pumping(IPC) are considered.We attribute the occurrence of the gain to the interaction of quantum coherence via decays and incoherent pumping.     

Electromagnetically Induced Grating Without Absorption Using Incoherent Pump

We propose a scheme for creating electromagnetically induced grating in a four-level double- Λ atomic system driven by a coupling field and an incoherent pump field. Owing to the incoherent pumping process, large refractivity accompanied with vanishing absorption or even gain across the probe field can be built up in the atoms, thus phase grating or gain-phase grating, which diffracts a probe light into different directions, can be formed with the help of a standing-wave coupling field. The diffraction efficiency of the gratings can be tuned by the coupling field intensity and the incoherent pump rate, hence the proposed gratings should be suitable for beam splitter and optical switching in optical communication and networking.     

Coherent aperture synthesis for incoherent source

We propose a refinement of the method for aperture synthesis for an incoherent source, imparting selectivity and noise immunity to the method. Instead of measuring the phase difference between the signals of the array receivers, we measure the array-averaged phase difference between neighboring receivers by searching for the maximum of the magnitude of the moving array’s response to the source signal during its direction-of-arrival-scanning (phasing). This procedure is carried out after incoherent accumulation. The gain in noise immunity is determined by the gain in the array with the addition of the gain in incoherent accumulation. The size of this gain in the experiment was on the order of 20 dB. The method retains applicability to wideband signals and the possibility of observing other incoherent signals. We demonstrate the possibility of high angular resolution with the synthesized antenna of the source, which does not possess the perfect time coherency of the source and other sources coherent with it. Owing to phase averaging over the array aperture, we obtain an increase in the method’s stability to reverberation. We present results of simulation and applicability of the method in full-scale experiments.     

Phase-dependent properties for absorption and dispersion in a closed equispaced three-level ladder system

It is shown that in a closed equispaced three-level ladder system, by controlling the relative phase of two applied coherent fields, the conversion from absorption with inversion to lasing without inversion (LWI) can be realized; a large index of the refraction with zero absorption can be gotten; considerable increasing of the spectrum region and value of the LWI gain can be achieved. Our study also reveals that the incoherent pumping will produce a remarkable effect on the phase-dependent properties of the system. Modifying value of the incoherent pumping can change the property of the system from absorption to amplification and enhance significantly LWI gain. If the incoherent pumping is absent, we cannot get any gain for any value of the relative phase.     

非相干背景光辐照对LiNbO3∶Fe与LiNbO3∶Fe∶In晶体中二波耦合的控制

理论分析和实验观测了非相干背景光辐照下掺杂LiNbO₃晶体中的二波耦合,发现非相干背景光能够在很大范围内灵活有效地控制信号光增益,抑制扇形效应,提高信噪比,缩短光栅的建立时间.入射光光强比一定时,适当地增大背景光光强可以使光栅响应时间减小一个数量级.尤其对于抽运光光强远大于信号光光强的情况下,上述作用更为明显. 关键词： 光折变 二波耦合 非相干背景光     

Phase control of probe response in an open ladder type system with spontaneously generated coherence

The control role of the relative phase between the probe and driving fields on the gain and dispersion in an open three-level ladder atomic system with incoherent pumping and spontaneously generated coherence is investigated. It is shown that by adjusting the value of the relative phase, the switching from lasing with inversion to lasing without inversion (LWI) can be realized; high index of the refraction with zero absorption can be achieved. It is also found that when the driving field is on resonance, the dispersion curve versus the probe detuning is very similar to the gain curve if the relative phase corresponding to the dispersion advances π/2 than that corresponding to the gain. LWI gain from the open system is obviously larger than that from corresponding closed system. When the incoherent pumping is absent, the open system can get LWI gain, moreover the gain is larger than that when the incoherent pumping presents for suitable value of the relative phase, but the corresponding closed system exhibits only absorption. In addition, manipulation effects of atomic exit and ratio of the injection rates on phase dependent gain are also analyzed.     

Effects of relative phase in an open ladder system without incoherent pumping

We studied effects of the relative phase between the probe and driving fields on the absorption and dispersion properties in an open three-level ladder system with spontaneously generated coherence but without incoherent pumping. It is shown that by the phase controlling, switching from absorption to lasing without inversion (LWI) and enhancing remarkablely LWI gain can be realized; large index of refraction with zero absorption and the electromagnetically induced transparency can be obtained. We also find that varying the atomic injection and exit rates has a considerable influence on the phase dependent-absorption property of the probe field, existent of the atomic injection and exit rates gives the necessary condition of the realization of LWI, getting LWI is impossible in the corresponding closed system without incoherent pumping.