推扫式多挡增益光谱成像模拟及噪声分析

综合性遥感监测对载荷的灵敏度和动态范围都有较高的要求,为此提出一种基于逐像元多挡增益切换的光谱成像方法。不同于帧增益或者行列增益切换的成像方式,该方法利用4T-APS CMOS探测器无破坏读出的特点,可以进行像元级的增益优化。这种方式可以兼顾水体、植被、云层等多要素的成像需求,极大地提高了载荷的研制效益。其基本原理为探测器先进行全局曝光,然后根据多级积分电容的饱和判断,选取不饱和的最高增益信号以增益码加信号的形式下传。地面根据增益码对应的定标系数反推出像元真实辐射值。由于谱段多且为分段响应,为保证系统的定量化应用,建立多挡增益光谱成像模型并进行噪声分析具有重要意义。通过对噪声类型的分析,建立了多挡增益下的泊松-高斯噪声模型。基于该模型计算了像元受噪声影响以低增益读出的概率。结果表明,虽然噪声会影响读出增益的变化,但影响区间极小,入瞳辐亮度在5 mW·cm-2·μm-1·sr-1以内,信号比增益挡位辐亮度分界值小0.05 mW·cm-2·μm-1·sr-1即可保证正常读出的概率大于99.6%。随着信号增强,光子噪声增大,增益减小,影响区间扩大。根据多挡增益信噪比模型,分析得出光谱模式与合并通道模式下的信噪比变化。最后,利用宽波段成像光谱仪（WIS）数据作为入瞳辐亮度进行了四挡增益的推扫式光谱成像模拟,分析了多挡增益光谱图像的固有特点。根据噪声模型对中心波长为0.443 μm的光谱图像添加1~3σ的随机噪声,分析了噪声对地物目标所处增益的影响。结果显示,在满足信噪比指标的前提下,该系统的单挡动态范围为74 dB,总动态范围可达114 dB。该方法不但提高了水体等弱信号的信噪比,而且可以保证建筑、云层等亮目标不饱和。成像模拟及噪声分析不仅有利于该载荷的后续研制,也可以为同类光谱仪器的设计提供参考。     

Statistical properties of laser-induced fluorescence signals

We point out the influence of the different noise sources which occur in the detection of the fluorescence signal induced by a laser in an atomic beam. We have developed a theoretical model which takes account of the atomic shot noise, photon noise, laser-frequency noise and a partition noise linked to the imperfect detection of the fluorescence photons. The calculations have been performed for two- and three-level atomic systems. We detail the own contribution of each noise source and give some predictions concerning the value of the fluorescence signal to noise ratio. We determine predominance domains of each noise source which depend on the values of key parameters such as the atomic flux intensity and the laser spectral linewidth. We particularly show that the laser-frequency noise, which induces a coupling between the emission of fluorescence photons by various atoms, leads to a saturation of the S/N ratio for intense atomic fluxes. Moreover, we point out that the optical pumping process associated with a three level atomic system leads to an interesting laser-noise filtering effect.     

The effect of radial radiation transport on the interpretation of saturation parameter measurements in laser systems

The influence of a spatial profile in the gain, which can be caused by saturation of the medium by the beam, is studied on the amplification properties of the beam. It is found that the amplifying medium is about twice as efficiently depleted as can be expected on basis of a plane-wave approximation. This means that the saturation-parameter values as they have been deduced in the past are wrong by a factor two. The theoretical results are applied to cw CO₂ measurements.     

Small-scale self-focusing of divergent beam in nonlinear media with loss

<正>Bespalov-Talanov theory on small-scale self-focusing is extended to include medium loss for a divergent beam.Gain spectrum of small-scale perturbation is presented in integral form,and based on the derived equations we find that the cutoff spatial frequency for perturbation keeps a constant value.The larger the medium loss is,the smaller the fastest growing frequency and the maximum gain of perturbation with defined propagation distance are.For a given medium loss the maximum gain of perturbation becomes larger,while the fastest growing frequency becomes smaller as the propagation distance becomes longer. Furthermore,physical explanations for the appearance of these features are given.     

非线性增益介质中强激光热像的演化规律

高功率激光系统中的热像效应可能导致光束的峰值功率剧烈增加,增益非线性介质会使这种光强增幅更为强烈。基于菲涅尔-基尔霍夫衍射理论和非线性近轴波动方程,对强激光在增益克尔介质工作在饱和区时的热像产生过程进行理论分析,将光束传输方程中增益饱和部分进行麦克劳林展开,取其近似,经过推导得出了介质薄近似时热像强度解析式和热像位置。通过数值模拟对解析结论预测的热像强度和位置进行验证。仿真结果表明,热像的位置在衍射物相对于介质对称处,热像强度解析结果与模拟结果相符,在薄介质时,解析解与模拟结果拟合较好。热像强度随非线性介质内非线性效应增强而停止增加,此外,讨论了热像强度随调制类型的变化。     

Quantum noise in optical beam propagation in distributed amplifiers

Quantum effects on optical beam propagation in a gain medium having distributed loss are analyzed using an amplifier-attenuator array model. For the amplifier-attenuator set, the Wigner function of the output field is related to that of the input field by a convolution law. Using this, we derive the Fokker-Planck equation for the beam propagation in the distributed amplifier with loss, and the added quantum noise is analyzed by solving this equation. When the amplification just compensates the attenuation, the mean amplitude of the signal does not change but the mean energy increases due to the added noise. Even in this case, the photon number variance can be reduced when a proper phase-sensitive reservoir is incorporated.     

波束域高分辨方位估计的方法、统计性能及水池实验结果

本文针对水声设备的实际应用，研究了波束域高分辨方位估计的MUSIC方法。首先分析了波束形成方法获得处理增益的原理，揭示了波束域高分辨方法具有较低分辨门限的机理；给出了波束域高分辨MUSIC算法，通过计算机仿真，比较了波束域和阵元域高分辨MUSIC法的统计性能，显示了波束域高分辨方法的优越性；进行了消声水池实验，结果表明波束域高分辨MUSIC法分辨力高，对阵列误差具有较大的宽容性。     

Analysis of the gain and noise characteristics of fibre Brillouin amplifiers

A theoretical analysis is performed of the transfer characteristics, gain, amplified spontaneous emission noise, on/off ratio and nonlinear phase shift of a fibre Brillouin amplifier, taking into account a possible signal detuning from the line centre and the nonlinear pump power depletion effect. Among other features, it is shown that the amplified spontaneous emission noise is reduced at higher signal levels and that detuning the signal reduces the amplifier gain in the linear regime and increases the spontaneous noise power in the saturation regime. As a consequence, the on/off ratio is significantly degraded by such an effect.     

Saturation behavior of a one-pump fiber optical parametric amplifier in the presence of the fourth-order dispersion coefficient and dispersion fluctuation

The influence of the fourth-order dispersion coefficient on the behavior of parametric gain and saturation power of a one-pump fiber optical parametric amplifier over a signal wavelength span in the presence of fiber random dispersion fluctuations was investigated. The output signal power for the parametric gain calculation was obtained by numerically solving the three-coupled amplitude equations. Based on the calculations of the parametric gain over a variation of the signal wavelength, it is found that the saturation power behavior is dependent on the behavior of parametric gain. The manipulations of signal wavelength and the fourth-order dispersion coefficient changed the phase-matching condition, thereby affecting the resulting parametric gain and saturation power.     

光参量啁啾脉冲放大增益特性研究

 通过对非线性三波耦合方程组进行数值求解，研究了光参量放大的增益特性及饱和放大特性。给出了泵浦光、信号光参数对参量增益的影响及其与晶体饱和长度的关系。且OPA具有传统CPA系统所不具有的优点，将有可能取代传统的CPA系统而作为超短超强激光系统的新型前端。     

Numerical modelling and experimental investigation of double-cladding erbium–ytterbium-doped fibre amplifiers

A comprehensive model for characterising double-cladding erbium–ytterbium-doped fibre amplifiers (EYDFAs) is proposed and experimentally verified for the first time to our knowledge. The model is based on rate and propagation equations in a homogeneous two-level active medium and takes into account the modified pump absorption arising from the double-cladding structure and the presence of ytterbium ions. The influence of the cladding pumping scheme on the multimode pump absorption, radial population inversion and the amplified spontaneous emission are also modelled. The numerical mode is capable to predict amplifier gain, output spectra, gain saturation and amplifier noise. The model has been fully validated in a large number of operating conditions and a fairly good agreement is obtained between the experimental data and the numerical simulations over a wide range of input parameters.     

Minimization of Cross-Gain Saturation in Wavelength Division Multiplexing by Optimizing Differential Gain in Semiconductor Optical Amplifiers

We successfully simulated the 10 × 40 Gbit/s soliton RZ-DPSK WDM signals over 1050 km with spectral efficiency approaching 0.4 bit/s/Hz using semiconductor optical amplifiers (SOAs) as in-line amplifier. The cross-gain saturation of SOA can be minimized by settling crosstalk at a lower level by decreasing the differential gain. This decrease in differential gain is in such a way that we get nil power penalty. The maximum transmission distance of 1050 km is possible with differential gain 210 atto cm² of SOA.

The impact of amplification factor, ASE noise power, crosstalk, quality factor and bit error rate for different differential gain has been investigated. It has been shown that with the increase in differential gain of SOA, the transmission distance goes on decreasing. At high value of differential gain 2.5 × 10^-16 cm² for the transmission distance 1050 km, all channels produce inter channel crosstalk with bit error rate greater than 10^-6. But for lower differential gain 190 atto cm², the quality of all channel increases at the cost of large power penalty.

With slight increase in differential gain 200 atto cm², the maximum transmission distance observed is 4550 km with quality of received signal more than 15 dB and having nil power penalty. We observed clear eye diagrams and optical power spectra for received signal with transmission distance 1050 km and 4550 km using soliton RZ-DPSK system. The bit error rate for all channels increase more than 10^-10 with the increase in launched input power that is due to power saturation.     

基于双程放大的毛细管放电69.8 nm激光增益特性

建立了类氖氩C线69.8 nm激光的双程放大实验后反射腔结构,利用45 cm长毛细管作为放电负载得到了其双程放大输出.在相同初始实验条件下,分别测量了单程放大输出与双程放大输出的激光脉冲光强、脉冲宽度以及激光束散角.通过对比单程与双程输出实验结果,利用双程放大激光光强的计算公式,分析得到了增益持续时间大于4 ns,以及增益在毛细管径向上的分布特点.以上结果为建立谐振腔进行毛细管放电类氖氩激光的多程放大实验奠定了基础.     

Low Noise-figure Miniature Erbium-doped Fibre Amplifier using Uncooled Pump Laser

A miniature-size erbium doped fibre amplifier (MS-EDFA) is constructed with a saturation power of 17.0 dBm and low noise figure of 5.2 dB using an uncooled pump laser. The MS-EDFA has achieved low polarization dependent gain, large signal gain and small signal gain of 0.05 dB, 15 dB (at Pin = +2 dBm) and 42 dB (at Pin = −30 dBm), respectively, with negligible temperature dependent gain. The dimension of MS-EDFA is only twice than a typical credit card.     

光扇效应对Ce∶KNSBN记录偏振组态的影响

在非同时读出条件下,采用Ar+ 514.5 nm单色激光为光源。以信号光为非寻常偏振光(e光),通过改变抽运光的偏振态,研究不同写入光偏振组态下Ce∶KNSBN晶体的两波耦合特性。结果表明     

入射光调制对Ce∶KNSBN晶体两波耦合的影响

研究了入射光调制条件下Ce∶KNSBN晶体两波耦合动态过程.结果表明,入射光调制抑制了光扇噪音对Ce∶KNSBN晶体两波耦合动态过程的影响,提高了透射信号光强度.同时研究了入射光光强比Ip/Is及入射光总光强Io对最佳调制频率及增益改善Gm/Gf的影响.结果显示,同一Io下,Ip/Is为100时,Gm/Gf到达峰值1.52,对应的最佳调制频率为150 Hz;同一Ip/Is下,Io为57 mW/cm2时,Gm/Gf最大为1.53,对应的最佳调制频率为175 Hz.     

All-optical logic gates based on two-photon absorption in semiconductor optical amplifiers

When the two-photon absorption of a high intensity pump beam takes place in a semiconductor optical amplifier there is an associated fast phase change of a weak probe signal. A scheme to realize fast all-optical XOR logic function using two-photon absorption induced phase change has been analyzed. Rate equations for semiconductor optical amplifiers, for input data signals with high intensity, configured in the form of a Mach–Zehnder interferometer has been solved. The input intensities are high enough so that the two-photon induced phase change is larger than the regular gain induced phase change. The model shows that both XOR operation and pseudo-random binary sequence generation at 250 Gb/s with good signal to noise ratio is feasible.     

典型概率密度背景下四阶累积量波束形成的阵增益

在理论上零均值高斯分布噪声的四阶累积量恒等于零,而实际噪声和混响的概率密度是影响四阶累积量波束形成技术性能的关键问题。本文针对海洋环境噪声与海底混响的瞬时幅度分布,分别推导了四阶累积量波束形成阵增益函数;建立了阵增益与阵元数、海洋环境噪声和海底混响统计特性的四阶矩和二阶矩及输入信噪比的关系;确定了与常规波束形成阵增益之间存在临界信噪比,高于临界信干比时四阶累积量波束形成可以获得比常规波束形成更高的阵增益与分辨率。实验数据处理结果验证了四阶累积量波束形成阵增益和分辨率理论结果的一致性。      

Effect of injection of C-band ASE on L-band erbium-doped fiber amplifier

The effect of injecting conventional band (C-band) amplified spontaneous emission on the performance of long-wavelength band erbium-doped fiber amplifier (L-band EDFA) is demonstrated. It uses a circulator and broadband fiber Bragg grating (FBG) to route C-band ASE from a C-band EDFA. Injection of a small amount of ASE (attenuation of 20 dB or above) improves the small signal gain with a negligible noise figure penalty compared to that of an amplifier without the ASE injection. A maximum gain improvement of 3.5 dB is obtained at an attenuation of 20 dB. At very large amounts of ASE injection (attenuation of 0 dB), the gain of the amplifier is clamped at 15.2 dB from ?40 to ?10 dBm with a gain variation of less than 0.3 dB. The saturation power is also increased from ?8 dBm (for without ASE injection) to 2 dBm (VOA=0 dB) with a slight noise figure penalty. These results show that the ASE injection technique can be used either for gain improvement or for gain clamping in L-band EDFA.     

微弱信号源的和波束定向方法与分裂波束定向方法的性能比较

本文讨论水下辐射噪声源的精确定向问题，给出被动声纳和波束定向与分裂波束定向方法的性能比较。指出在一定信噪比下，分裂波束精确测向技术比和波束定向技术具有较高的定向精度。但是，随着信噪比的下降，两者趋于一致。推导了估计定向精度的分析表达式，给出在直线阵和圆弧阵情况下，延时估计和声源入射角偏差之间的换算公式和数值模拟结果。同时给出数字式声纳用以计算入射信号左波束和右波束数据的互谱来实现分裂波束定向的方法。