All-optical switching at ultralow light levels

We report an experimental demonstration of all-optical switching at ultralow light levels in coherently prepared Rb atoms. A signal light pulse is switched on and off by a control light pulse at different frequencies in a four-level atomic system based on multiphoton interferences. We observed a switching efficiency of 55% with the signal and control light pulses containing approximately 20 photons each, corresponding to a control energy density of approximately 10(-5) photons per atomic cross section lambda(2)/(2pi).     

Simulations of measurements of fluorescence lifetimes using noise-modulated light

At the Pattern Recognition group at the Delft University of Technology, we are working on new ways to measure fluorescence lifetimes. There are two well-known ways to measure lifetimes; the phase method and the pulse method. In the phase method fluorescent material is stimulated by sinusoidally modulated light. The emitted fluorescent light will have the same modulation frequency, but there will be a phase shift between the excitation and the emission light. Measuring this phase shift will, after some simple calculation, give the lifetime of the fluorescent material. The second method is the pulse method. Short pulses of light are used to excite the material. The emitted light is detected, and from these measurements the decay curve of fluorescent light is determined. In our research we want to use a new method that may allow us to measure a mixture of lifetimes. We want to use excitation light that is modulated by a white noise signal. We are currently building an experimental setup for these measurements. We have been working on numerical and electrical simulations to investigate the properties of noise signals. Some results of these simulations are presented in this paper.     

Controlling dielectric properties of cBN by an ultrashort double-pulse light

The optical responses of wide band gap materials can be manipulated by a purposely designed ultrashort double-pulse light, based on the real-time time-dependent density functional theory. Taking the cubic boron nitride (cBN) as a typical example, we calculate the energy transfer from an ultrashort double-pulse light field to dielectric materials. We show that the purposely designed ultrashort double-pulse light and pulse train can reversibly manipulate the dielectric properties of cBN without damage. By tuning the parameters of light pulse, the cBN will change between isotropic, elliptic and hyperbolic for a particular light frequency at certain time points during the pulse light. We propose that the cBN with reversible but deep nonlinear optical responses excited by the external light field can be used in the ultrafast all-optical signal processing and optical-field-effect devices.     

Reduction of light signal pulse distortion in cascaded sum-frequency-generation and difference-frequency-generation wavelength conversion

Cascaded sum-frequency-generation (SFG) and difference-frequency-generation (DFG) can implement a wavelength conversion between arbitrary combinations of input and output signal wavelengths. By using a tunable wavelength pump light, the output wavelength can be tuned to a desired wavelength. As in many wavelength conversion devices using the nonlinear optical effect, the group velocity difference between light pulses with different wavelength causes a walk-off effect deforming the output pulse shape. Thus the device length should be kept short to avoid the walk-off effect resulting in limited conversion efficiency. In this report, we propose a method for a quasi-phase matched (QPM) device to maintain the pulse shape of the SFG light pulse along the propagation distance. The output DFG light pulse deformation is suppressed and the conversion efficiency can be increased by extending the device length.     

液晶空间光调制器光谱整形的理论研究

 将液晶空间光调制器用于超高斯种子脉冲光谱整形,重点分析了调制器参数对压缩脉冲信噪比的影响,以此为依据对其结构参数进行优化设计,以降低其影响。对整形、放大及压缩过程的理论模拟表明:应用液晶空间光调制器进行光谱整形时,能够有效克服钕玻璃啁啾脉冲放大系统的非线性效应,在放大器输出端保持原始光谱宽度,但液晶空间光调制器对光谱的调制作用使压缩脉冲信噪比降低。     

Slow and fast light based on SBS with the spectrum tailoring

In this paper, slow and fast light of a signal pulse of nanosecond duration are achieved in the same experimental device by using band-pass filters to tailor the spectra of spontaneous Brillouin scattering (SPBS) coming from 50 km single-mode optical fibers (SMF) as the pump light. The phase matching condition of slow and fast light is satisfied by changing wavelengths of band-pass filters .In our experiment, we obtain the tunable delay time for various signal pulses range from 1 ns to 20 ns. By increasing the power of the pump field to 16 mW, a 1 ns signal pulse is delayed 1.5 ns and is advanced 1.0 ns respectively, while the signal pulse almost has no distortion. The experiment has demonstrated that larger relative pulse delay of slow and fast light can be obtained in the same experimental setup when pump power is lower.     

Controllable beating signal using stored light pulse

We experimentally study the controllable generation of a beating signal using stored light pulses based on electromagnetically induced transparency(EIT) in a solid medium. The beating signal relies on an asymmetric procedure of light storage and retrieval. After storing the probe pulse into the spin coherence under the EIT condition, two-color control fields with opposite detunings instead of the initial control field are used to scatter the stored spin coherence. The controllable beating signal is generated due to alternative constructive and destructive interferences in the retrieved signal intensities. The beating of the two-color control fields is mapped into the beating of weak probe fields by using atomic spin coherence. This beating signal will be important in precise atomic spectroscopy and fast quantum limited measurements.     

三能级原子系统中单光子频率失谐对光减速的影响

在Λ型三能级Rb原子介质中，观察到了由电磁感应透明（EIT）效应导致的光减速现象并测 量了光延迟对单光子频率失谐量的依赖关系. 结果表明，由于多普勒展宽效应的存在，在单 光子频率失谐±600MHz的范围内，光减速效应较为显著. 在考虑多谱勒频移的情况下，数值 计算了光延迟随单光子频率失谐量的变化曲线，实验结果与理论曲线很好地符合. 这一研 究结果为利用单光子频率失谐控制光的群速度提供了理论与实验参考. 关键词： 光减速 电磁感应透明 多普勒展宽     

信号光的调制对基于光纤光参量放大光脉冲源的改善

在理论上从四波混频的强度耦合方程出发,给出了基于光纤光参量放大(FOPA)光脉冲的光场表达式,并进一步分析了信号光被相位调制或强度调制后,光脉冲的频率啁啾和强度演化.结论指出：若信号光被强度调制,对所生成脉冲宽度无明显的影响,但可以提高消光比;若信号光被相位调制,所生成的脉冲具有更大的线性正啁啾,可以在相同的抽运功率条件下得到比无相位调制时更窄的脉冲.实验上给出了10 GHz工作速率下的结果,其结果与理论分析符合得很好.通过信号光的相位调制,在05 W平均抽运功率条件下得到了消光比22 dB,脉宽为5 关键词： 光纤参量放大 四波混频 光脉冲源 频率啁啾     

Two-photon and two-photon-assisted slow light

We show that light pulses propagating in two-photon absorbing systems may present time delays like slow light produced via coherent population oscillations in one-photon interactions. Two regimes are numerically studied for a simplified two-level system: (a) a light pulse at frequency ω/2 undergoes two-photon absorption (TPA) and is delayed by the absorbing system (two-photon slow light) and (b) a light pulse at frequency ω is delayed in a system prepared by TPA of a light pulse at frequency ω/2 (two-photon-assisted slow light). The study carried out in solutions of dyes and dendrites shows significant delays, low distortion, and good transmission for easily reachable experimental conditions. The working principle can be applied to other media and can be used in telecommunications technology.     

Ultra-high on/off ratio optical trigger based on SOI–SOA waveguide

We present a device able to perform an all-optical trigger by utilizing the cascaded silicon-on-insulator (SOI) and semiconductor-optical-amplifier (SOA) optical waveguides, where both the controlling pulse and the signal pulse are co-propagating along the device. The results of numerical simulations show that the all-optical trigger behavior with ultra-high on/off ratio can be easily achieved by adjusting the device parameters. To improve and optimize the optical trigger operation, the negative initial delay (the signal pulse before the controlling pulse) and the positive initial delay (the signal pulse after the controlling pulse) are considered for performing the optical trigger operation.     

Storage of light in atomic vapor

We report an experiment in which a light pulse is effectively decelerated and trapped in a vapor of Rb atoms, stored for a controlled period of time, and then released on demand. We accomplish this "storage of light" by dynamically reducing the group velocity of the light pulse to zero, so that the coherent excitation of the light is reversibly mapped into a Zeeman (spin) coherence of the Rb vapor.     

基于瞬态光栅频率分辨光学开关法测量飞秒脉冲的研究

超宽光谱的飞秒脉冲测量一直是超快激光领域的重要研究方向之一.常规的飞秒脉冲自相关方法是通过测量自相关倍频信号来获得,而倍频信号具有波长选择性,不同中心波长的飞秒脉冲测量需要更换不同的倍频晶体,十分不方便.因此,提出了一种改进型的瞬态光栅频率分辨光学开关(TG-FROG)方法用于测量飞秒脉冲.该方法结合四波混频和频率分辨光学开关方法,其基本过程是将待测脉冲分为三束,其中两束脉冲经过精密的延时控制并聚焦在光学介质上达到时空重合,利用三阶非线性效应产生稳定的瞬态光栅作为开关光;另一束脉冲作为探测光与产生的瞬态光栅进行相互作用产生一个信号光,使用光谱仪对该信号光的光谱与延迟时间进行测量,并通过反演迭代算法处理而获取待测飞秒脉冲的光谱与电场信息.该方法只需要待测光的功率密度达到三阶非线性效应就可以实现测量,因此可以应用于任意中心波长的飞秒脉冲测量.利用该方法对中心波长分别为800 nm, 400 nm的飞秒脉冲,以及超连续亚10 fs的周期量级超宽光谱飞秒脉冲进行了测量,并与常规的干涉自相关仪器测量结果进行了比较,所得测量结果基本一致.实验结果表明,建立的基于TG-FROG方法对不同中心波长,不同脉冲宽度的飞秒脉冲测量是十分有效的.     

Photons trapping within a nano-ring resonator controlled by light

We propose the interesting results that a bright and dark soliton pulse can be localized within a nonlinear nano-waveguide. The system consists of nonlinear micro- and nano-ring resonators, whereas the soliton pulse can be input into the system and trapped within the nano-waveguide. A soliton input is chopped by the nonlinear effects known as chaos into smaller pulses. The required pulse is filtered and amplified, which can be controlled and localized within the nano-waveguide. The localized bright and dark solitons are trapped within a nano-waveguide by controlling the nano-waveguide input power, which means that the photons trapping is controlled by light.     

Signal velocity, causality, and quantum noise in superluminal light pulse propagation

We consider pulse propagation in a linear anomalously dispersive medium where the group velocity exceeds the speed of light in vacuum ( c) or even becomes negative. A signal velocity is defined operationally based on the optical signal-to-noise ratio, and is computed for cases appropriate to the recent experiment where such a negative group velocity was observed. It is found that quantum fluctuations limit the signal velocity to values less than c.     

矩形谱宽带光抽运的布里渊慢光中脉冲失真的分析

在小信号增益条件下推导出了高斯光脉冲经过矩形谱宽带光抽运的布里渊慢光系统后输出脉冲时域振幅包络的近似解析表达式,适用于抽运光频谱具有陡峭上升(下降)沿和平坦顶部的情况.定量分析和比较了光纤色散以及受激布里渊散射增益不均衡和增益所致色散效应对延迟信号光脉冲失真的影响.计算结果表明:当信号光脉冲的频谱处于布里渊有效增益谱内时,解析解与数值解符合,布里渊增益所致三阶色散效应是导致延迟信号光脉冲失真的主要物理因素,制约了窄脉冲延迟量的提高.     

The efficiency of parallel quantum memory for light in a cavity configuration

We present a new scheme of quantum memory for optical images (spatially multimode light fields) that allows mapping the quantum state of the signal onto the long-lived coherence of the ground state of an ensemble of stationary atoms or impurity centers. The memory medium is embedded in an optical cavity with degenerate transverse modes, which increases the effective optical thickness of the medium and allows one, in principle, to store information in optically thin atomic layers. Since, in reality, storage and retrieval of limited-duration signals, including signals shorter than the lifetime of the field in the cavity, is of interest, we do not use the low-Q cavity approximation. The influence of losses due to partial reflection of the nonstationary signal field incident on a coupling mirror on the storage efficiency is considered. We used the method of approximate impedance matching, wherein losses due to reflection can be minimized by controlling the coupling parameter of the light field with memory medium in time, thus creating conditions for destructive interference of the signal and local fields on the coupling mirror. The influence of diffraction on the transverse resolution of memory at the writing and readout stages is investigated, and the number of effectively stored transverse spatial modes of the signal is estimated.     

Slow light with integrated gain and large pulse delay

We demonstrate slow and stored light in Rb vapor with a combination of desirable features: minimal loss and distortion of the pulse shape, and large fractional delay (>10). This behavior is enabled by (i) a group index that can be controllably varied during light pulse propagation, and (ii) controllable gain integrated into the medium to compensate for pulse loss. Any medium with the above two characteristics should be able to realize similarly high-performance slow light.     

Slow and stored light in an amplifying double-Lambda system

We show analytically and numerically that a double-Lambda system, characterized by parametric amplification of cross-coupled probe and four-wave mixing pulses, is an excellent medium for producing both slow and stored light. Moreover, this system can compensate for the absorption and broadening processes that usually occur without destroying the coherent properties of the medium. The stored probe signal pulse is written by means of a simple Lambda system and is retrieved and amplified, at the same frequency, by a double-Lambda system that also produces a pulse at the four-wave mixing frequency.     

Driving light pulses with light in two-level media

A two-level medium, described by the Maxwell-Bloch system, is engraved by establishing a standing cavity wave with a linearly polarized electromagnetic field that drives the medium on both ends. A light pulse, polarized along the other direction, then scatters the medium and couples to the cavity standing wave by means of the population inversion density variations. We demonstrate that control of the applied amplitudes of the grating field allows one to stop the light pulse and to make it move backward (eventually to drive it freely). A simplified limit model of the Maxwell-Bloch system with variable boundary driving is obtained as a discrete nonlinear Schr?dinger equation with tunable external potential. It reproduces qualitatively the dynamics of the driven light pulse.