飞秒激光用多层介质膜脉宽压缩光栅的设计

以薄膜光学的干涉理论和衍射光学的傅里叶模式理论为基础，给出了0.8μm飞秒激光器用多层介质膜脉宽压缩光栅的理论设计；设计采用H3L(HL)^9H0.5L2.4H的多层介质膜为基底，当刻蚀后表面浮雕结构的占宽比为0.35，线密度为1480线/mm，槽深为0.2μm，顶层HfO₂的剩余厚度为0.15μm时，对于Littrow角度(36.7°)和TE波模式入射的衍射光栅其-1级衍射效率达到95％以上. 关键词： 飞秒激光 脉宽压缩光栅 多层介质膜     

Effect of Dielectric Groove Profiles on Reflection Characteristics of Millimeter Wave Frequency Selective Surface

In this paper, the frequency selective reflection characteristics of dielectric periodic structures with arbitrary groove profiles are analyzed by a method which combines the multimode network theory with the rigorous mode matching procedure. Emphasis is placed on the discussion about the effect of the different groove profiles, such as rectangle, trapezoid, increasing cosine and triangle, on the reflection characteristics of the millimeter wave (MMW) frequency selective surface (FSS). Some useful guidelines for the design of the MMW dielectric FSS are thus suggested. Moreover, the physical explanation of the related wave phenomenon is given.     

脉宽压缩光栅用多层膜制备和性能测试

给出了以413 nm作为写入波长,1053 nm作为使用波长的多层介质光栅膜的设计,计算表明膜系H3L(H2L)^9H0.5L2.03H满足光栅膜的要求.最后给出了制备得到的样品光学特性测试,结果表明在使用波长和曝光波长处满足设计要求,其抗激光损伤阈值在光正入射和51.2°入射时分别为14.14 J/cm2和9.32 J/cm2,膜系的应力表现为压应力.     

脉宽压缩光栅用的多层膜设计和性能分析

应用于啁啾脉冲放大技术中的脉宽压缩光栅是基于多层膜作为基底,利用全息干涉技术和离子束技术刻蚀而成。脉宽压缩光栅的衍射效率和抗激光损伤阈值一方面依赖于光栅结构的设计,另一方面很大程度上取决于作为基底的多层膜的设计。给出了以413.1nm作为写入波长,1053nm作为使用波长的多层介质光栅膜的设计,样品在ZZS-800F型真空镀膜机上采用电子束蒸发方式沉积而成,并给出了膜系结构对光学性能影响因素的详细分析,结果表明膜系H3L(H2L)9H0.5L2.03H满足光栅膜的指标。给出了样品光学特性测试,其使用波长处的透射率<0.5%,写入波长处的透射率>90%,测试表明样品满足设计要求且实验结果和理论设计符合得很好。     

Investigation of the Dielectric-Loaded, Ridged Helical Groove Slow-Wave System for the Millimeter Wave TWT

A dielectric-loaded, ridged helical groove slow-wave structure for the millimeter wave traveling wave tube is presented in this paper. The effects of the groove depth, ridge dimensions and the dielectric permitivety on the wave propagating properties and the interaction impedance are investigated in detail. From the analysis, it is indicated that for broader band amplification in a helical groove travelling wave tube, a ridged helical structure with shallow groove and loaded by dielectric with proper relative permitivety may be applied.     

Methode d''excitation du spectre Raman d''une couche mince utilisant un reseau a grand nombre de traits

The Raman spectrum of a dielectric substance deposited in a few microns thick film on a diffraction grating (aluminium coating), is obtained with a trapped laser beam inside the film. In order to assure the most efficient coupling, a single mode of propagation is excited in “reverse coupling”; the selection of groove spacing is discussed.     

Analysis of Radiation Phenomena in a Wedge Ended Dielectric Slab Waveguide

Radiation phenomena observed in a wedge shape ended dielectric slab waveguide are analyzed using mode matching technique. The case of transverse electric polarization (TE) being parallel to dielectric slab waveguide is assumed. In order to describe the fields in the wedge region, a stack of dielectric plates is assumed and in each layer the fields are expanded in terms of the mixed spectrum of guided and radiated modes. A similar expansion is used in the constant thickness slab waveguide while in free space medium a continuous-radiation mode expansion is used. Then a mode matching approach is applied, incorporating the orthogonality properties of mixed spectrum modes, in order to compute the wave fields inside the dielectric slab waveguide and wedge medium. Mode matching is achieved by discretizing the continuous radiation mode spectrum leading into a numerically stable solution provided a sufficient large number of points are used to convert integrals into finite summations. Numerical computations are carried out for various wedge geometries and shapes including linear and exponential profiles.     

Enhanced laser induced damage threshold of dielectric antireflection coatings by the introduction of one interfacial layer

A new method for increasing laser induced damage threshold (LIDT) of dielectric antireflection (AR) coating is proposed. Compared with AR film stack of H2.5L (H:HfO2, L:SiO2) on BK7 substrate, SiO2 interfacial layer with four quarter wavelength optical thickness (QWOT) is deposited on the substrate before the preparation of H2.5L film. It is found that the introduction of SiO2 interfacial layer with a certain thickness is effective and flexible to increase the LIDT of dielectric AR coatings. The measured LIDT is enhanced by about 50%, while remaining the low reflectivity with less than 0.09% at the center wavelength of 1064 nm. Detailed mechanisms of the LIDT enhancement are discussed.     

Double Gires-Tournois interferometer negative-dispersion mirrors for use in tunable mode-locked lasers

We report the implementation and operation of novel superhigh-reflectivity negative-dispersion dielectric mirrors for use in tunable ultrafast laser systems. The mirror structure is divided into two distinct regions: an underlying superhigh-reflectivity dielectric quarter-wavelength stack and an overlying negative-dispersion section consisting of only a few layers and forming simple multiple Gires-Tournois interferometers. The example that we present was designed for operation from 800 to 900 nm and has a near-constant group-delay dispersion of -40 fs(2) and a peak reflectivity greater than 99.99%. We show a comparison of the predicted and the measured mirror performance and application of these mirrors in a mode-locked Ti:sapphire laser tunable from 805 to 915 nm.     

LEAKY CHARACTERISTICS OF A NEW MILLIMETER WAVE ANTENNA BASED ON GROOVE GUIDE WITH AN ASYMMETRIC CONDUCTOR STRIP

A new millimeter wave leaky wave antenna is presented in this paper, which is a modification of the groove guide with an asymmetric conductor strip by introducing a dielectric slab into the conductor plane region of the groove guide antenna. The leakage property of the new antenna is carefully examined through calculations of an equivalent network, which is developed here with taking into account the coupling between different modes to guarantee the calculating accuracy. It has been found that the new leaky wave antenna is of larger leakage constant than that for the original groove guide antenna because of the introduction of the dielectric slab. Extensive numerical results of the leakage characteristics are given to establish useful guidelines for the design of the new type groove guide leaky wave antenna.     

向列相液晶染料可调谐激光器的研究

对向列相液晶染料的可调谐激光器进行了光学特性研究. 以650 nm为中 心波长设计了SiO₂和TiO₂多层膜的一维光子晶体, 以激光染料与向列相液晶的混合物作为增益介质层, 制备了波长可调谐激光器.用Nd: YAG倍频脉冲激光器输出的532 nm激光抽运所制备的激光器样品得出如下光学特性: 激光发射波长随温度调谐范围为605.5---639.8 nm, 达到34.3 nm, 随电压调谐范围为634.5---619.5 nm, 达到15 nm. 发射激光每脉冲的阈值能量为12.3 μJ, 激光线宽小于1 nm.     

Analysis and design of grating couplers

Based on an accurate perturbation analysis of the guiding properties of dielectric gratings, simple design criteria are developed for grating couplers which transfer the energy of a beam into or out of an optical waveguide. Gratings having arbitrary groove shapes are considered and explicit formulae are given for the leakage parameters of gratings with symmetric profiles. The results cover TE_v and TM_v modes and they apply to both shallow and deep grating grooves. The variation of the leakage parameter α in rectangular gratings is examined in detail; these rectangular gratings are then used as basic configurations for predicting the characteristics of other grating profiles. Particular attention is given to trapezoidal and triangular profiles and gratings with asymmetric profiles are also discussed.     

Nanofabrication of a space-variant optical transmission filter

A space-variant optical transmission filter is demonstrated for which a simplified process is used to tailor the spatial response of the filter across the surface of a single wafer. A multilayer stack, of alternating high or low refractive index dielectric materials, was used to produce a narrow transmission notch in the center of a wide stop band. Subsequent patterning and etching of arrays of holes through the volume of the dielectric stack was performed to control the fill factor of the dielectric in the layers. The position of the transmission notch within the reflection spectrum was varied across the device surface by adjusting the hole diameter of the hole arrays. Experimental and numerical simulation were used to confirm the space-variant transmission characteristics of a single-wafer sample with two zones of different hole diameter arrays in the 1550 nm wavelength regime.     

一种控制矩形光刻胶光栅槽深和占宽比的方法

利用光刻胶的非线性效应可以制作出了矩形的全息光栅。制作矩形光栅时,对槽形的控制被简化为对槽深和占宽比这两个参量的控制。首先借助实时潜像监测技术获得最佳曝光量,然后根据显影监测曲线的特征找出光栅槽底的残胶厚度为零的显影时刻,就能得到槽底干净的矩形光栅,同时保证槽深近似等于光刻胶的初始厚度;如果此后继续显影,就能适度减小占宽比。实验结果和理论分析都证实了这种控制方法的可靠性。对1200lp／mm的光栅,目前工艺能精确调控的最大槽深为1μm,占宽比在0．2～0．6范围内。实验还揭示,为了提高对光栅槽形的调控能力,必须首先提高干涉条纹的稳定性。     

Dielectric effect on the rf characteristics of a helical groove travelling wave tube

A new type of partial-dielectric-loaded helical groove slow-wave structure (SWS) for millimetre wave travelling wave tube (TWT） is presented in this paper.The radio-frequency characteristics including the dispersion properties,the longitudinal electric field distribution and the beam-wave coupling impedance of this structure are analysed.The results show that the dispersion of the helical groove circuit is weakened,the phase velocity is reduced and the position of the maximum Ez is moved from the mouth to the inside of the groove after partially filling the dielectric materials in the helical groove SWS.Therefore,the dielectric-loaded helical groove SWS is suitable for a multi-beam TWT with broad band and high gain.     

Broadband and high efficiency metal multi-layer dielectric grating based on non-quarter wave coatings as reflective mirror

This article deals with designing broadband and high efficiency metal multi-layer dielectric grating (MMDG) used to compress and stretch ultra-short laser pulse. The diffraction characteristics of MMDG are analysed with the method of rigorous coupled-wave method. The multi-layer dielectric used as reflective mirror is made up of non-quarter wave coatings. Taking the diffraction efficiency of the -1 order as the value of merit function, the parameters such as groove depth, residual thickness, duty cycle, and reflective mirror are optimized to obtain broadband and high diffraction efficiency. The optimized MMDG shows an ultra-broadband working spectrum with the average efficiency exceeding 97% over 160 nm wavelength centred at 1053 nm and TE polarization. The optimized MMDG should be useful for chirped pulse amplification.     

Polarization effect of femtosecond pulse breakdown in subwavelength antireflective relief grating

The transmittance property and the near-field distribution of subwavelength broadband antireflective grating directly patterned into the wide bandgap dielectric material as a function of the surface period and groove depth are performed by a rigorous Fourier modal method. It is found that the transmittivity is insensitive for TE and TM polarization, but the near-field distribution associated with laser damage resistance ability is strongly dependent on polarization state of incident light. What's more, the femtosecond pulse laser damage threshold of surface structure taking into account local maximum electric field enhancement was calculated numerically using a theoretical ionization mechanism model. The higher threshold on the surface period, pulse duration and incident wavelength for TM polarization than that for TE wave is demonstrated quantitatively.     

Broadband and high efficiency metal multi-layer dielectric grating based on non-quarter-wave coatings as a reflective mirror

This article deals with designing a broadband and high efficiency metal multi-layer dielectric grating (MMDG) used to compress and stretch an ultrashort laser pulse. The diffraction characteristics of the MMDG are analysed by using the rigorous coupled-wave method. The multi-layer dielectric used as the reflective mirror is made up of non- quarter-wave coatings. Taking the diffraction efficiency of the 1 order as the value of merit function, the parameters such as groove depth, residual thickness, duty cycle, and reflective mirror are optimized to obtain broadband and high diffraction efficiency. The optimized MMDG shows an ultra-broadband working spectrum with the average efficiency exceeding 97% over 160 nm wavelength centred at 1053 nm and TE polarization. The optimized MMDG should be useful for chirped pulse amplification.     

F2-laser ablation patterning of dielectric layers

Spatially defined patterning of multi-layer dielectric optical systems by laser-induced ablation is demonstrated. A 49-layer high-reflectivity mirror for 193-nm light was irradiated with F₂-laser light through the CaF₂-substrate to cleanly remove the whole dielectric stack by rear-sided ablation. The 157-nm light is absorbed efficiently by dielectric layers such as SiO₂ and Al₂O₃ that are typically used for ultraviolet (UV) transmission at 193-nm and longer wavelengths. Thus it is possible to ablate highly reflective UV-laser mirrors (HR 193 nm) and to create dielectric masks that withstand high power levels at 193 nm. A single 157-nm pulse with a fluence of less than 500 mJ/cm² is sufficient to cleanly ablate the whole layer stack with sharp edges and without debris deposition. Received: 31 October 2000 / Accepted: 14 November 2000 / Published online: 10 January 2001     

High packing density laser diode stack arrays using Al-free active region laser bars with a broad waveguide and discrete copper microchannel-cooled heatsinks

A high packing density laser diode stack array is developed utilizing Al-free active region laser bars with a broad waveguide and discrete copper microchannel-cooled heatsinks. The microchannel cooling technology leads to a 10-bar laser diode stack array having the thermal resistance of 0.199 ℃/W, and enables the device to be operated under continuous-wave （CW） condition at an output power of 1200 W. The thickness of the discrete copper heatsink is only 1.5 mm, which results in a high packing density and a small bar pitch of 1.8 mm.