Signal-to-noise ratio improvement of a supercontinuum continuous-wave optical source using a dispersion-imbalanced nonlinear optical loop mirror

A multiwavelength continuous-wave (cw) optical source is based on supercontinuum (sc) produced in a highly nonlinear fiber. Noise properties of this optical source are studied through numerical simulations based on the nonlinear Schrödinger equation. The numerical simulations show that the amplified spontaneous emission (ASE) generation in an erbium-doped fiber amplifier and the ASE amplification during the sc generation result in a serious degradation of the signal-to-noise ratio (SNR). A novel scheme for improvement of the SNR of the multiwavelength cw optical source based on sc by using a dispersion-imbalanced nonlinear optical loop mirror is presented and the numerical simulation results are given. As a result, the average level of the SNR improvement is 11 dB and the eye-opening penalty is reduced by 0.91 dB due to the SNR improvement. In addition, the scheme can also be applied for improvement of the SNR of a pulse-type multiwavelength light source. PACS 42.50.Lc; 42.65.Re; 42.79.Sz; 02.60.Cb     

调制不稳定性对级联放大光纤传输系统信噪比的影响

在考虑光纤损耗和级联放大器的情况下，推导了调制不稳定性的产生条件和增益的普适解析表达式，分析了调制不稳定性对信噪比的影响，给出了一个新的计算信噪比的表达式。     

干涉成像系统信噪比的统计分析

根据统计光学建立了干涉成像系统信噪比的理论模型,数值分析了干涉成像系统信噪比与系统参数之间的关系。分析结果表明:为了提高系统的信噪比,应提高信号光场强度与噪声光场强度之比,使参考光和测试光光强接近相等,降低系统中光学元件的数量和每个元件的偏差,使光源相干长度约为测试光和参考光间光程差的2～5倍。要使信噪比大于10,则要求系统中光学元件总偏差的标准差小于1/4波长。信噪比的统计理论模型可为干涉成像系统设计和干涉测量提供理论指导。     

Optimization of signal-to-noise ratio in integrated planar optical interconnects packages

The signal-to-noise ratio (SNR) of integrated planar optical (IPO) board-to-board interconnections system is analyzed. The SNR is derived as a function of separation between the boards, beam spot size, optical system efficiency, receiver thermal noise, thickness of the substrate, and the angle of propagation of the beam. Based on the analyses and the results obtained it is shown that the design of optical interconnects system can be optimized for maximum SNR using at least one parameter, namely, angle of propagation.     

星间微波光子链路调制方式的优化

针对外调制星间微波光子链路输出信噪比优化问题,建立了基于双电极马赫 曾德尔调制器的强度调制直接探测星间微波光子链路模型,通过优化调制器调制方式来提高链路性能。用数值模拟方法得到了单边带、双边带和推挽式3种调制方式下链路输出信噪比,利用曲面投影法求得了最优调制方式时一定信噪比要求下发射端所需最小光放大器增益和对应的调制器直流偏置相位。结果表明:相同输入射频信号功率和发射光功率情况下,双边带调制输出信噪比比单边带调制高3 dB,低直流偏置相位推挽调制可以进一步优化输出信噪比。输入射频信号功率为-20 dBm,输出信噪比为17.3 dB时,所需最小光放大器增益为43.9 dB,对应的直流偏置相位为0.87。     

窄带傅里叶变换光谱仪中平稳高斯噪声的理论分析

在本文所研究的空间调制型傅里叶变换光谱仪中, 在窄的光谱带宽内进行探测可以有效提高光谱的分辨率. 为了研究光源辐射噪声对系统的影响, 本文将辐射噪声归纳为一种高斯窄带的平稳随机过程, 利用线性系统分析方法, 根据统计学原理推导了辐射噪声作为随机变量通过干涉系统前后的输入信噪比与输出信噪比. 然后, 对于一个具有窄带矩形光谱的系统进行了计算仿真, 得出了信噪比增益随着光程差在不同自相关度和互相关度取值时的变化情况. 仿真结果表明, 不同光程差下的信噪比增益在相关度空间是一单调的平滑曲面, 增益极值始终沿着相关度圆的半径和周线移动, 并且经过一个光程差又回到初始的位置. 根据对信噪比增益的分析, 可以将噪声的相关度取值控制在某一范围之内, 并作为系统光源设计与测试的依据.     

Improvement of signal-to-noise ratio in digital holography using wavelet transform

The basic problem in optical and digital holography is the presence of speckle noise in the reconstruction process, which reduces the signal-to-noise ratio (SNR). The presence of speckle noise is serious drawback in optical and digital holography since it substantially reduces the SNR in the reconstructed image. In this paper, we present wavelet filtering to improve SNR in the reconstructed images from digital holograms. Experimental results are presented.     

基于无衍射光的光互连系统的对准偏差研究

以信噪比为性能指标,以相邻光通道之间的串扰和探测器前置放大器的热噪声作为系统噪声的来源,研究了基于无衍射光的光互连系统在不同对准偏差条件下的性能.研究表明,探测器阵列的尺寸参数可作为系统对准偏差容忍度和信噪比的优化参数.扩大探测器半径,减小探测器间距可以使系统包容更大的对准偏差,但为了保证一个可以接受的信噪比,探测器间距不应过小.此外,采用无衍射光的光互连系统具有不受纵向对准偏差影响的优点.     

Analytical expressions for momentum autocorrelation function of a classic diatomic chain

We use recurrence relations method to study a classical harmonic diatomic chain. The momentum autocorrelation function results from contributions of acoustic and optical branches. By use of convolution theorem, analytical expressions for the acoustic and optical branches are derived as even-order Bessel function expansions. The expansion coefficients are given in terms of integrals of real and complex elliptic functions for the acoustic and optical branches, respectively. Double convolution results respectively in integrals of trigonometric and hyperbolic functions for expansion coefficients of acoustic and optical branches.     

Long-distance fiber Bragg grating sensor system with a high optical signal-to-noise ratio based on a tunable fiber ring laser configuration

A novel tunable fiber ring laser configuration with a combination of bidirectional Raman amplification and dual erbium-doped fiber (EDF) amplification is proposed for realizing high optical signal-to-noise ratio (SNR), long-distance, quasi-distributed fiber Bragg grating (FBG) sensing systems with large capacities and low cost. The hybrid Raman-EDF amplification configuration arranged in the ring laser can enhance the optical SNR of FBG sensor signals significantly owing to the good combination of the high gain of the erbium-doped fiber amplifier (EDFA) and the low noise of the Raman amplification. Such a sensing system can support a large number of FBG sensors because of the use of a tunable fiber Fabry-Perot filter located within the ring laser and spatial division multiplexing for expansion of sensor channels. Experimental results show that an excellent optical SNR of approximately 60 dB has been achieved for a 50 km transmission distance with a low Raman pump power of approximately 170 mW at a wavelength of 1455 nm and a low EDFA pump power of approximately 40 mW at a wavelength of 980 nm, which is the highest optical SNR achieved so far for a 50 km long FBG sensor system, to our knowledge.     

Modeling and optimizing the pulse replicator based on the active recirculating optical loop

Signal to noise ratio (SNR) is an important parameter in the output signal of the pulse replicator. We propose a novel new method to improve this parameter. Instead of focusing on lowering the noise level of the optical amplifier and limiting the accumulation of amplified spontaneous emission (ASE) noise in the loop, we consider the optimization of other system parameters. By modeling with a predefined implementation of the pulse replicator, an evaluation function of SNR is constructed, with variables such as the coupling ratio, the optical fiber length. In this paper, we will construct the quantitative function between SNR and the coupling ratio. According to the evaluation function, we can determine the optimal value of the coupling ratio for the coupler to obtain the best SNR within the predefined parameters. The experiment will prove that the SNR in the output signal of the pulse replicator has been increased while using the optimal coupling ratio. This will contribute greatly to future work on improving SNR of the pulse replicator.     

Optimal alignment sensing of a readout mode cleaner cavity

Critically coupled resonant optical cavities are often used as mode cleaners in optical systems to improve the signal-to-noise ratio (SNR) of a signal that is encoded as an amplitude modulation of a laser beam. Achieving the best SNR requires maintaining the alignment of the mode cleaner relative to the laser beam on which the signal is encoded. An automatic alignment system that is primarily sensitive to the carrier field component of the beam will not, in general, provide optimal SNR. We present an approach that modifies traditional dither alignment sensing by applying a large amplitude modulation on the signal field, thereby producing error signals that are sensitive to the signal sideband field alignment. When used in conjunction with alignment actuators, this approach can improve the detected SNR; we demonstrate a factor of 3 improvement in the SNR of a kilometer-scale detector of the Laser Interferometer Gravitational-Wave Observatory. This approach can be generalized to other types of alignment sensors.     

Linearity of quadrant avalanche photodiode in laser tracking system

The tracking precision of laser tracking system is affected by the angular resolution of quadrant avalanche photodiode. According to the detecting principle of quadrant avalanche photodiode, the light spot area, the optical intensity distribution, the non-uniformity of response, and the signal-to-noise ratio （SNR） that affect the linearity of the detector are studied. The light optical spot area and the optical intensity distribution can be adjusted through software. The non-uniformity of response and the SNR are influenced by the noise of the detector. Because the noise is affected by the optical intensity of the incident laser, it is difficult to obtain the law of the linearity caused by noise. When the light spot area and the optical intensity distribution are fixed, the other factors can be measured. With the detector scanned in raster scanning mode, the non-uniformity of response is measured at different SNRs. The linearity of detector is measured by a moving target that can reflect the illuminating laser to the detector in diffuse reflection mode. The nonlinear error of the linearity of detector can be minimized by increasing the SNR.     

基于白光LED照明光源的室内VLC系统

LED照明与可见光通信技术相结合,构建出基于LED可见光无线通信系统。对室内VLC(Visible Light Communication)系统的白光LED光源特性和系统信道模型分析,提出照明光源布局设计与接收光功率分布的关系;对强度调制直接检测方式的室内VLC系统中信噪比和多径效应引起的码间串扰分析,提出采用光分集接收技术克服码间串扰和提高信噪比,并给出光检测器阵列布局的模型。建立VLC系统仿真模型,给出OOK-NRZ (On-Off Keying & Non-Return Zero)和OOK-RZ(On-Off Keying & Return Zero)调制方式的误码率和均方根时延扩展之间的关系曲线。仿真结果表明,接收光功率相同时,均方根时延扩展时间大于1.0 ns时,OOK-RZ特性优于OOK-NRZ。     

Optical Multistability and Wavelength Switching of a Fabry-Perot Semiconductor Laser with Multi-Detuned Light Injections

Optical multistability of a Fabry-Perot semiconductor laser is analyzed when multi-optical inputs detuned from the cavity-resonant wavelength of a semiconductor laser are injected. Many split branches are shown to be produced in the optical output versus optical input characteristics for different detuned optical inputs. It is also shown that optical outputs can be switched in any one of the optical input wavelengths by optical input pulses, based on the optical output versus input characteristics with multi-split branches.     

Improved signal-to-noise ratio in spectral-domain compared with time-domain optical coherence tomography

A signal-to-noise ratio (SNR) analysis is presented for optical coherence tomography (OCT) signals in which time-domain performance is compared with that of the spectral domain. A significant SNR gain of several hundredfold is found for acquisition in the spectral domain. The SNR benefit is demonstrated experimentally in a hybrid time-domain-spectral-domain OCT system.     

相关多普勒光学层析成像

提出了基于互相关的多普勒OCT(correlated Doppler optical coherence tomography, CD-OCT)方法, 能够有效的抑制噪声, 实现低信噪比条件下的流速探测. 对CD-OCT算法进行了详细的推导, 分析了噪声的相关性对该算法结果的影响, 最后基于谱域和时域联合探测方法(joint spectral and time domain optical coherence tomography, STD-OCT)以及CD-OCT算法的对比实验证明了该算法能够进一步实现信噪比的提高, 使测量的结果更为稳定.     

Analysis of a partially coherent optical correlator in the presence of phase defects at the input plane

The effects of phase defects at the input plane of an optical correlator on the performance of the system under partially coherent illumination is investigated. The correlation signal-to-noise ratio (SNR) is evaluated as the criterion for the performance of the optical system. We study the effects of both spatially and temporally partially coherent illumination on the distorted correlation signal. It is shown that the output SNR can be increased by relaxing the coherence requirements of the system. The SNR dependence on noise variation is also investigated. It is found that the correlator becomes less sensitive to noise variations under partially coherent illumination.     

基于多光谱遥感成像链模型的系统信噪比分析

信噪比(SNR)是评价多光谱遥感成像性能的重要指标,在设计多光谱遥感成像仪的最初阶段应进行分析,从而确定各分系统相关参数。多光谱遥感系统的成像链模型综合考虑辐射源、地物光谱反射、大气辐射传输、光学系统成像、分光元件特性、探测器光谱响应和相机噪声等各个环节,可用于进行成像过程端对端的完整分析。以基于滤光片阵列的多光谱遥感系统为例,采用MODTRAN软件进行大气辐射传输计算,对不同太阳天顶角下,不同目标地物计算像面的照度,根据电荷耦合器件探测器的噪声模型,计算出不同工作条件下多光谱遥感系统的SNR。通过对SNR的分析,可给出该类型多光谱遥感系统获得最佳性能的工作条件,并能够结合使用要求进行光学系统参数的优化选择。     

Improvement of the optical signal-to-noise ratio in common-path holographic storage by use of a polarization-controlling media structure

A holographic storage arrangement with common-path optics is likely to suffer from degradation of the optical signal-to-noise ratio (SNR) owing to scattering of the reference beam. Instead of the conventional method of spatial separation between the reference beam and a reconstruction beam, adoption of a combined spatial and polarization separation is proposed. To achieve polarization separation by using a photopolymer as the recording material, a media structure including retardation films and a reflection layer is proposed such that the polarization states of the two beams are the same inside the material but orthogonal outside. Preliminary experiments to show the feasibility of this idea for improving optical SNR are also demonstrated.