薄膜截止滤光片的消偏振设计

薄膜截止滤光片在倾斜入射时不可避免地会产生s和p二个偏振分量的分离,因而在许多应用,特别是光通讯的应用中成为一个棘手的难题。提出了一种新的设计方法,对最常用的45°入射角,实现了长波通和短波通两种截止滤光片的完全消偏振, 在透射率为50%处,其偏振分离分别为0.3 nm和0.1 nm。基本的设计方法是采用宽带法布里珀罗薄膜干涉滤光片中心波长两侧的干涉带作为长波通或短波通截止滤光片的初始膜系,然后经过适当的优化以提高透射带的透射率。宽带干涉滤光片的间隔层常由半波长厚度的高、低折射率混合膜层组成,如2H2L2H或2L2H2L。由于这种设计的截止区和透射带带宽常嫌不足,故提出了展宽截止区和透射带的方法。对一个典型的短波通截止滤光片,在波长1550 nm,截止区和透射带宽均达到了200 nm。这种设计方法不仅简单、性能优良,而且膜厚控制容差较大,故易于制造。     

矩形波宽带通滤光膜研究

对矩形波宽带通滤光片进行了深入研究,提出了一种设计、制备矩形波宽带通滤光片的方法。使用该方法设计并制备了400 nm～1 100 nm波段,中心波长λ₀=515 nm,透射带λ=λ₀±25 nm,透射带平均透射率■≥92%,截止带λ=400 nm～475 nm、λ=555 nm～1 100 nm,截止带透射率小于0.1%的矩形波宽带通OD3-A滤光片。对样片光谱进行了测试,结果满足需求。该方法设计、制备矩形波宽带通滤光片克服了F-P型窄带滤光膜监控精度要求高、通带宽带窄、成本高以及传统长、短波截止膜组合方式膜层总厚度过大、通带透过率低、波形矩形度差的缺点。     

用于投影显示的分色合色膜系的消偏振设计

由于斜入射时,光学薄膜存在一定的偏振效应,将产生S-和P-偏振光的光谱分离.在Philips棱镜系统中,分色合色膜系通常分色时对S-光应用,合色时则对P-光应用.从提高光能利用率、减少杂散光和增加系统对比度等因素综合考虑,要求分色合色膜系的S-和P-偏振光的分离尽可能小.新设计方法采用宽带法布里－珀罗薄膜干涉滤光片中心波长两侧的干涉带作为长波通或短波通截止滤光片,可实现S-和P-偏振分量的分离几乎为零.     

光学薄膜技术讲座 第四讲 干涉截止滤光片

一、引言要求某一波长范围的光束高透射,而偏离这一波段的光束骤然变化为高反射(或称抑制)的干涉截止滤光片有着广泛的应用。通常我们把抑制短波区、透射长波区的滤光片称为长波通滤光片,相反,抑制长波区、透射短波区的截止滤光片就称为短波通滤光片,干涉截止滤光片的基本类型是上一     

硅薄膜的短波红外光学特性和1.30μm带通滤光片

在短波红外区域(1～3μm),硅薄膜材料因其具有折射率高、透明性好、膜层应力易匹配等诸多优点而得到广泛应用。基于改进后的Sellmeier模型拟合出了制备的硅薄膜的短波红外光学特性,以此为基础,选用硅和二氧化硅两种材料,设计并制备出中心波长在1.30μm,相对带宽2.46%的带通滤光片。利用了硅薄膜在波长小于1.0μm波段的吸收特性较好地扩展了带外截止范围。测量结果表明,具有2个谐振腔的带通滤光片峰值透射率达到85.8%,半功率带宽控制在约32nm,带外截止范围覆盖了波长小于1.75μm的光谱区域。     

光学薄膜特性的锥角效应数值分析

建立非平行光入射多层膜的模型,给出了在圆形光束锥角入射时的薄膜特性表达式,对三类典型光学薄膜即多腔滤光片、非偏振分光薄膜和非偏振截止滤光薄膜的特性进行了数值实验。研究结果表明:随着光束入射锥角的增加,多腔滤光片的影响是中心波长向短波方向移动,透射率下降和通带形状退化;非偏振分光薄膜的s和p偏振透射率增加,宽带波纹度增加;非偏振截止滤光薄膜的通带透射率出现周期振荡,s偏振的通带波纹度大于p偏振,通带波纹度总体具有增加的趋势,过渡区的陡度具有变大的趋势。     

角度调谐滤光片特性分析及膜系设计

对窄带滤光片的倾斜入射特性作了分析.斜入射时其透射通带和峰值会向短波方向移动,透射曲线的稳定性跟滤光片的间隔层结构相关,多腔间使用相同的间隔层可以保证斜入射时有稳定的峰值透射率和带宽,利用该特点可以制备角度调谐窄带滤光片.透射光S和P偏振分量的中心波长随入射角度的增大出现分离现象,产生较大的偏振相关损耗.通过搭配不同厚度的高低折射率材料作为间隔层,改变其有效折射率,使其两个偏振分量的中心波长实现重合.设计了符合密集波分复用(Dense Wavelength Division Multiplexing,DWDM)系统要求的低偏振相关损耗四腔窄带角度调谐滤光片膜系,其可调谐范围达20 nm以上,并评估了所设计角度调谐滤光片的调谐性能.     

一种大入射角容差宽带薄膜偏振器

通过组合长波通和短波通膜堆的方法,设计并制备了一种大入射角容差宽带薄膜偏振器。该薄膜为HfO_2/SiO_2结构,采用无离子束辅助的电子束蒸发工艺,蒸发金属铪和SiO_2制得。对该膜的透射率光谱、激光损伤阈值和形貌进行了研究,结果显示其不仅在1044~1084 nm波段内都具有很高的对比度,而且在1064 nm波长、53°~60°的入射角范围内具有很大的消光比和激光损伤阈值,且损伤特性基本不随入射角变化。该偏振器的P光损伤阈值约为20 J/cm~2,损伤主要由基板与薄膜界面处的纳米级缺陷所引起;S光损伤阈值约为45 J/cm~2,损伤主要由激光辐照下薄膜表面的等离子烧蚀现象引起。     

激光敌我识别系统中滤光膜的研制

根据激光敌我识别系统的使用要求,选择H4和SiO2作为高低折射率材料,借助Macleod和TFCalc软件进行膜系优化设计和分析。采用电子束真空镀膜的方法并加以离子辅助沉积技术,通过正交矩阵实验对材料的工艺参数进行调整和优化,利用基片的正反面分别对带通滤光片的长波反射带和短波反射带进行展宽,解决了单面膜层过厚难以控制的问题,制备了符合要求的激光滤光膜。镀膜后的基片在532,632,905,1064和1550nm波长处的透射率小于0.2%,808nm波长附近的平均透射率大于95%。并在532nm和1064nm波长处具有较高的激光损伤阈值,能承受恶劣的环境测试,满足激光敌我识别系统中光学仪器的使用要求。     

双折射消偏振膜的设计和制备

基于倾斜沉积薄膜材料的双折射特性,采用单一TiO2设计和制备了中心波长为632 nm的双折射消偏振膜。首先以60°和70°的沉积角度镀制了TiO2单层膜,通过单层膜的透射光谱分别拟合出两种沉积角度下薄膜对s和p偏振光的等效折射率nPH,nSH和nPL,nSL。通过对折射率的组合,实现正入射时s偏振光透射率大于p偏振光,而入射角度增大会使两者透射率差值减小,基于这一思想设计并制备了消偏振膜。分别测量了消偏振薄膜在400~800 nm波段范围内正入射及倾斜入射条件下s和p偏振光的透射光谱。入射角为60°时,s偏振光反射带宽基本与p偏振光反射带宽重合,在波长632 nm左右基本实现消偏振。结果表明,利用材料的双折射特性,可以设计和制备出倾斜入射条件下消偏振薄膜。     

Theoretical analysis and simulation of conjugate heights for dual-conjugate AO system in lidar

A multi-conjugate adaptive optics （MCAO） can offer a possibility of widening field of view （FOV） characterized by the isoplanatic angle, and the choose of conjugate height becomes a basic problem for MCAO, which influences the size of iosplanatic angle. Considering the application of lidar, the isoplanatic angle＇s expressions of two deformable mirrors （DMs） MCAO for uplink and downlink are deduced. The effects of conjugate heights for dual-conjugate AO are thoughtfully discussed, and the isoplanatic angles are further analyzed. The results show that the isopanatic angle varies with the conjugate height and reaches the maximum as the conjugate height is at the optimal altitude. Moreover, the optimal conjugate height changes with the propagation distance.     

Quick single-photon detector with many avalanche photo diodes working on the time division

Due to the limit of response speed of the present single-photon detector, the code rate is still too low to come into practical use for the present quantum key distribution (QKD) system.A new idea is put up to design a quick single-photon detector.This quick single-photon detector is composed of a multi-port optic-fiber splitter and many avalanche photo diodes (APDs).Au of the ports with APDs work on the time division and cooperate with a logic discriminating and deciding unit driven by the clock signal.The operation frequency lies on the number N of ports, and can reach N times of the conventional single-photon detector.The single-photon prompt detection can come true for high repetition-rate pulses.The applying of this detector will largely raise the code rate of the QKD, and boost the commercial use.     

Characterization of PolyA and PolyC mismatches by Raman spectroscopy

A.C mismatches are studied by Raman spectral characterization of PolyA, PolyC, and their equimolar complex in solution of 0.14 mol/L Na ,pH7.0.Experimental results show that A·C mismatches occur to be A/B (mainly A) conformers, and unlike Watson-Crick base pairing, this kind of mismatches is stabilized by only one hydrogen bond involving cytosine N4H2 and adenine N7.The formation of A·C complex makes the base stacking interactions much stronger, and conformation of the backbone more ordered, which leads to obvious Raman hypochromic effect with some shifts in corresponding bands.     

Improvement of four-wave mixing-based wavelength conversion efficiency in dispersion shifted fiber by 40-GHz clock pumping

40-GHz clock modulated signal as a pump to improve the efficiency of four-wave mixing (FWM)-based wavelength conversion in a 26.5-km dispersion shifted fiber (DSF) is investigated. The experimental results demonstrate that the conjugated FWM component has higher intensity with the clock pumping than that with the continuous-wave (CW) light pumping. The improvement of FWM-based wavelength conversion efficiency is negligible when the pump power is less than Brillouin threshold. But when the pump power is greater than Brillouin threshold, the improvement becomes significant and increases with the increment of pump power. The improvement can increase up to 9 dB if pump power reaches 17 dBm.     

Multi-pulse operation of a Kerr-lens mode-locked femtosecond laser

Our experimental results show that the presence of a proper amount of negative group velocity dispersion is essential to multi-pulse operation of a Kerr-lens mode-locked femtosecond laser. We demonstrate that the pulse separations and the number of pulses contained within a cavity round trip are strongly dependent on the initial perturbations. The results allow us to get a better understanding on the influences of the convoluted self-phase modulation and intra-cavity dispersions on the stable multi-pulse oscillation in a Kerr-lens mode-locked femtosecond laser.     

Propagation of Helmholtz-Gauss beams in weak turbulent atmosphere

Based on the Rytov approximation of light propagation in weak turbulent atmosphere,the closed-form expressions of field and average irradiance of each one of the four fundamental families of Helmholtz-Gauss (HzG)beams:cosine-Gauss beams,stationary Mathieu-Gauss beams,stationary parabolic-Gauss beams,and Bessel-Gauss beams,which are propagating in weak turbulent atmosphere,are obtained.The results show that the field and average irradiance can be written as the product of four factors:complex amplitude depending on the z-coordinate only,a Ganssian beam.a factor of complex phase perturbation induced by atmospheric turbulence,and a complex scaled version of the transverse shape of the non-diffracting beam.The effect of weak atmospheric turbulence on irradiance distribution of the HzG beam can be ignored.     

An effective method for reducing speckle noise in digital holography

An effective method for reducing the speckle noise in digital holography is proposed in this paper.Different from the methods based on classical filtering technique,it utilizes the multiple holograms which are generated by rotating the illuminating light continuously.The intensity images reconstructed by a series of holograms generated by rotating the illuminating light possess different speckle patterns.Hence by properly averaging the reconstructed intensity fields,the speckle noises can be reduced greatly.Experimental results show that the proposed method is simple and effective to reduce speckle noise in digital holography.     

Influence of purity of HfO2 on reflectance of ultraviolet multilayer

The impurities in two kinds of HfO2 materials and in their corresponding single layer thin films were determined through glow discharge mass spectrum technology and secondary ion mass spectrometry(SIMS) equipment respectively.It was found that ZrO2 was the main impurity in the two kinds of HfO2 either in the original HfO2 materials or in the electron beam deposited films.In addition,the difference of Zr content in the two kinds of HfO2 single laver films was much laxger than that of the other impurities such as Ti and Fe.which showed that it was just ZrO2 that made the difference between the optical performance of the film products including the two kinds of HfO2.With these two kinds of HfO2 and the same kind of SiO2.we deposited HfO2/SiO2 multilayer reflective coatings at the wavelength of 266 nm.Experimental results showed that the reflectances of these two mirrors were about 99.85% and 99.15% respectively,which agreed well with the designed results what were based on the optical constants obtained from the corresponding single layer thin films.     

Speech enhancement by array crosstalk resistant ANC and spectrum subtraction

Microphone array-based speech enhancement has great importance for speech communications and speech recognition. To reduce the aperture of the microphone array and to increase the effect of the speech enhancement will greatly broaden the application areas of the microphone array. An array crosstalk resistant adaptive noise cancellation method is therefore presented. And then an improved spectral subtraction algorithm is further cascaded to obtain better enhancement results. Theoretic analysis and experiments indicate that the proposed scheme needs only a very small microphone array while it simultaneously achieves a higher SNR improvement. Besides, the proposed scheme can be used in many noisy environments and is easy for real-time implementation.