【作者】 陈麟；

【导师】 曹庄琪；

【作者基本信息】 上海交通大学 ， 光学， 2008， 博士

【摘要】 双面金属包覆波导结构是一种特殊类型的光波导结构。与传统的介质波导结构相比,它对电磁场的约束能力更强,而且导模的有效折射率范围更大,可从零开始,这样,不用借助于棱镜或光栅等耦合器件,从自由空间就可以将电磁场能量耦合进波导。由于金属作为波导覆盖层将对传输光能量产生吸收,导致波导损耗较大,所以将双面金属包覆波导用作反射型器件来克服传输型结构损耗过大的缺点。双面金属包覆波导结构在传感器,滤波器,电光调制器等领域获得了广泛的应用。本文在前人工作的基础上,重点探讨在双面金属包覆波导衰减全反射峰附近增强的古斯汉欣效应。同时,我们也分析了双面金属包覆波导超高阶导模的色散性质并提出一种实现波长传感和稳定的新方法。本文介绍了电磁场在激发表面等离子波结构和介质波导结构中的传输机理,推导了它们的反射率公式和本征方程。由于入射场和波导内本征场之间要实现模式耦合,所以常用棱镜实现传播常数的匹配。而双面金属包覆波导的导模传播常数的范围比其他波导结构大的多,这样就很容易从自由空间将场的能量耦合进波导,这种特点对器件的小型化十分有利。本文详细介绍了双面金属包覆波导中的超高阶导模的奇特性质和应用。这种超高阶导模具有偏振无关,衰减全反射谱分立,对导波层厚度、折射率和波长的变化灵敏以及慢波特性,在窄带滤波器,梳状滤波器,溶液浓度传感器,位移和加速度传感器,电光调制器等领域获得了广泛的应用。在本文中,我们详细研究了双面金属包覆波导的损耗特性,包括辐射损耗和本征损耗,基于一阶微扰方法推导出了它们的定量公式。辐射损耗来源于耦合层的厚度有限,电磁场可以通过消逝场和外界交换能量,它由耦合层金属膜的厚度决定。本征损耗来源于金属包覆层的吸收性质,它由金属介电系数的虚部决定。在此基础上,通过静态相位法,采用弱耦合近似,我们推导出当传播常数匹配时,波长变化下的双面金属包覆波导反射光的侧向位移的简洁公式。从这个公式可以看出,侧向位移的符号取决于共振模的本征损耗与辐射损耗。当本征损耗大于辐射损耗时,侧向位移取负值,反之为正,并且本征损耗与辐射损耗之差越小,古斯汉欣位移的绝对值就越大。我们通过高斯光束法验证了这个结论,并理论得到双面金属包覆波导反射光的侧向位移可增强到几百微米。本文提出了一种测量双面金属包覆波导侧向位移的实验方法。这种方法基于可调谐激光器和一维位置灵敏探测器。可调谐激光器所发出的激光经过分束镜和小孔,入射在待测光波导的上层金属膜上。波导样品放置在倍角转台上。由计算机编程控制的倍角转台可以进行角度扫描。从波导反射的光首先被光电探测器接收,并转变为电压信号输入到计算机数据采集卡中。选择入射角度,使得在相邻的吸收峰之间反射率最大。由于偏离共振条件,在这个角度侧向位移并不明显,反射光斑的位置可被视为基准。然后将光电探测器从光路中移开,同时不改变入射光角,让反射光直接垂直入射到位置灵敏探测器的中心。通过调节激光器的温度改变入射光的波长,位置灵敏探测器两侧的信号经放大得到的输出电压输入到计算机,对相关数据作计算处理就可获得所需要的侧向位移值。通过实验,我们发现了最高可达480μm的正的侧向位移和180μm的负的侧向位移。实验结果和理论值符合的很好。接着,基于这种效应,我们提出了一种新的测量溶液浓度的方法。这种方法的分辨率理论上可达10-9数量级,比采用表面等离子共振结构的相关方法的分辨率高一到两个数量级。本文还提出了一种基于双面金属包覆波导超高阶导模测量波长微小变化的传感器,并在高斯光束下分析和讨论了它的灵敏度,计算了波导的最优设计参数。这种传感器可以获得很窄的反射吸收峰,其灵敏度显著高于传统的波长探针,实验中测得灵敏度达5×1010m-1。这种传感器还具有制作工艺简单,与偏振无关等优点,可广泛应用于波长锁定和波长监测等领域。更多还原

【Abstract】 Symmetrical metal-cladding waveguide (SMCW) is a special type of optical waveguide. Compared to traditional dielectric waveguide, it has the stronger confining effect and the wider range of effective refractive index of guided modes which begins from zero. According to this character, without other coupling techniques, such as prism coupling and grating coupling, a so called free space coupling technique is developed to transfer energy into waveguide. Because the transmission loss of the waveguide will be extremely large if metal films are used as cladding layers, which is bad for producing useful devices. We prefer to the reflection manner rather than transmission manner, so the transmission loss of energy is avoided. Many opti-electronic devices are introduced based on SMCW, including sensor, filter and modulator. In the dissertation, the enhanced Goos-H?nchen (GH) effect of a reflected beam from SMCW, and the dispersion characteristics of SMCW are extensively discussed. Then a new method is proposed to stabilize and monitor wavelength.Firstly, the principle of the Surface Plasma Resonance (SPR) and dielectric waveguide are described. Their reflectivity and dispersion equation are derived. Because the incident mode must coincide with the synchronous condition, Prism is commonly used to match longitudinal wavenumber. The range of effective refractive index of guided modes on SMCW is much larger than other dielectric waveguide. Free space coupling technique is developed to transfer energy into SMCW. This characteristic is useful for the small size of devices. We demonstrate that the ultra-high order modes in SMCW with submillimeter scale are polarization independent and sensitive to the refractive index, thickness of the guided layer and the incident wavelength. Some typical applications of the SMCW, such as narrow band filter, comb filter, oscillating wave sensor, displacement sensor and electro-optical modulator, are also mentioned.Secondly, the loss characteristic of SMCW, including the intrinsic and radiative damping, is discussed and derived based on the first order perturbation theory. The intrinsic damping results from imaginary part of metal dielectric constant and represents absorption loss of the guided wave due to the materials. The radiative damping represents the leakage loss of the guided mode back into free space and is inversely proportional to the exponential function of thickness of upper metal layer. Using stationary-phase approach and weak coupling condition, a concise equation of GH shift is obtained when incident wavelength coincides with the synchronous condition. From this equation, we can obtain that the magnitude of the beam shift is closely related to the intrinsic and radiative dampings of the resonant mode. When the intrinsic damping is larger than the radiative damping, negative GH shift occurs. The positive GH shift corresponds to the reverse case. Larger GH shift can be obtained when intrinsic damping approaches the radiative damping. This conclusion is also proved by Gaussian beam model. The shifts can be enhanced to as large as hundreds of micrometers.Then we present an elegant experimental approach to directly measure the GH shift for a single total reflection of the light beam from SMCW. A one-dimensional position-sensitive detector (PSD) that can provide small spatial resolution is used to detect the large GH shift of the reflected beam when the ultrahigh-order mode is excited in the SMCW. After passing through two apertures and a splitter, a large part of the Gaussian beam from a tunable laser is introduced onto the SMCW with upper metal layers of different thicknesses. Another part of the beam, which is reflected from the first splitter, irradiates the second splitter and is detected by a wavemeter connected to a computer. In the experiment, ultrahigh-order modes are excited. Because of the polarization independence of the ultrahigh-order modes, TE and TM incidence have nearly the same characteristics. The reflected light from the SMCW is first detected by a photodiode. An angular scan is performed by rotating the goniometer and the spectrum is generated. We select the operation angle to be located at the maximum reflectivity near a certain dip of the spectrum. The GH effect is not remarkable at this position due to the deviation of the resonance condition. The position of the reflected beam is set as the reference at this point. Then we move the photodiode out of the light path without changing any position of the incident beam and let the reflected light beam cast onto the center of the PSD perpendicularly. Then by changing the wavelength through temperature tuning, the variation of GH shift can be measured. The positive and negative shifts about 480 and 180μm can be observed, respectively. The trend of experimental results shows good agreement with the numerical results. Furthermore, a new sensor is proposed by measuring the ultrahigh-order mode enhanced GH shift. The resolution of this sensor can reach 10-9 RIU theoretically, which is higher than SPR-GH sensors.Finally, a novel wavelength sensor based on ultrahigh-order mode from SMCW is also proposed. The sensitivity of the ultrahigh-order mode and the optimal thickness of the upper gold thin films are discussed. Owing to the very narrow resonance dip, the proposed device exhibits unusual sensitivity enhancement. Its sensitivity is much higher than traditional wavelength probes and can be obtained as high as 5×1010m-1 experimentally. This sensor has quite excellent application prospect owing to its many advantages, such as high sensitivity, simple structure and independence of polarization, and can be widely used to stabilize and monitor wavelength.更多还原