Modeling Long-Pass Filters Based on Fundamental-Mode Cutoff in Photonic Crystal Fibers

Index-guiding fibers presenting a Ge-doped photonic-crystal cladding and a pure-silica core are numerically studied for the development of efficient optical long-pass filters. They present a fundamental-mode cutoff (FMC) toward short wavelengths that leads to a high extinction ratio, sharp band edge, and bend-tunable transmission profile. The simple design poses no experimental challenge for fabrication and the relatively large obtained modal area makes these waveguides fully compatible with standard telecommunication fibers. The proposed long-pass filter compares favorably with standard, bulk, technology, and with a previously demonstrated photonic crystal fiber presenting a FMC.     

Analysis of Thermo-Optic Tunable Dispersion-Engineered Short-Wavelength-Pass Tapered-Fiber Filters

Thermo-optic tunable short-wavelength-pass tapered-fiber filters based on fundamental-mode cutoff mechanism are realized experimentally and analyzed theoretically. The effects of material and waveguide dispersion are investigated and the optimal tapered fiber structures for attaining high-spectral cutoff slope and high-rejection efficiency are determined.      

低折射率芯PCF基模损耗的数值模拟与分析

应用多极法理论计算了低折射率芯光子晶体光纤（PCF）基模的损耗并与未掺杂芯PCF进行了比较，发现可以从损耗的角度来理解低折射率芯PCF的截止特性：传输波长靠近短波长时，模式的损耗会突然变大，从而导致基模截止；而处于长波长时，这种光纤的传输特性和普通PCF相类似。通过改变纤芯折射率的大小和包层中空气孔的大小，可以对光纤的截止波长进行调节。     

以单轴晶体为内包层的双包层光纤偏振特性研究

本文研究了以单轴晶体为内包层的双包层高双折射光纤的传输和偏振特性．在弱波导近似下,应用光波导理论,得出了ｏ光与ｅ光的色散方程及截止特性．通过计算机的模拟与分析,划分出了截止区、单模单偏振区、单模双偏振区及多模区;分析了单模单偏区和单模双偏区存在的条件;研究了单模双偏区两偏振模式的偏振色散特性．结果表明,几何参数Ｓ、光学参数Ｒｘ、Ｒｙ及外包层折射率η３对所划分的四个区域及所对应的偏振特性有着极为明显的影响．此结果为光纤保偏器的设计和应用高双折射单模光纤构成新型光纤传感器的设计提供了可靠的理论依据．     

Semiconductor laser with tapered-rib adiabatic-following fiber coupler for expanded output-mode diameter

A new expanded-mode AlGaAs diode laser with reduced far-field divergence and 0.9 dB single-mode optical fiber coupling loss is demonstrated. This new laser achieves a two-dimensional (2-D) expansion of the fundamental output mode with a rib waveguide tapered in only one dimension. Stable fundamental-mode operation with a narrow, 5.7/spl deg/ by 7.40 full-width-at-half-maximum far-field divergence is observed well above lasing threshold. Tapered waveguide dimensions are compatible with conventional optical lithographic techniques, making this device suitable for mass production.     

Wideband tunable Gaussian-shaped spectral filters based on dispersion engineering

Optical liquids can be used to engineer the dispersion characteristics of fibers by serving as the core or cladding to attain fundamental-mode cutoff effect. The short-/long-pass fiber filters are so made and concatenated to achieve widely thermo-optic tunable Gaussian-shaped spectral filters. The proposed wideband tunable Gaussian-shaped spectral filter provides a potential technique in application to high resolution bio-imaging.     

Adiabatic coupling in tapered air-silica microstructured opticalfiber

We study adiabatic mode propagation in tapered air-silica microstructured optical fibers and demonstrate efficient coupling into a robust high-delta microstructured fiber. In the waist region of the taper, the core mode is tightly confined by the air holes and exhibits properties similar to a high-delta waveguide such as enhanced peak intensity and widely flattened anomalous dispersion. We exploit these properties to generate tunable self-frequency shifting Raman solitons over the communications window from 1.3 μm to 1.65 μm, with over 60% conversion efficiency These fiber devices are practical for several reasons: they can be fusion spliced to standard single-mode fibers with relatively low loss, they are mechanically strong, due to the supporting cladding, and because the mode is isolated from the surrounding air interface, they can potentially be recoated allowing for packaging     

Chasi-Guided Modes and Related Radiation Losses in Optical Dielectric Waveguides with External Higher Index Surroundings

Mode filter actions are found theoretically in an optical dielectric waveguide consisting of a core and a thin cladding layer which is further surrounded by an external higher index region. The propagating waves, which are usually considered to be cutoff modes, can be guided with a small amount of loss under a certain condition. These waves are defined here as quasi-guided modes. These modes tend to the guided modes of the guide when the cladding thickness increases infinitely. A method is given to estimate the losses. As an example, the radiation losses are formulated for a symmetric slab waveguide, and are found to be approximately proportional to the cube of the mode number of the quasi-guided mode. Therefore, losses of the quasi-guided modes depend strongly on the mode number. It is suggested that fibers with large core diameters can be used as quasi-single mode fibers by covering the clad-type multimode fibers with external higher index surroundings and choosing the parameters properly.     

应用等效折射率模型研究光子晶体光纤

应用等效折射率模型研究了光子晶体光纤(PCF)的传播特性.介绍了光子晶体光纤的等效折射率模型.通过求解标量波动方程得到了光子晶体光纤包层基空间填充模的模式折射率,利用阶跃光纤的理论来研究光子晶体光纤的导模特性.应用此模型对不同结构光子晶体光纤包层区的等效折射率与波长的关系进行了讨论.包层区等效折射率与芯子的折射率差随波长的增加而增大,并由此阐述了光子晶体光纤的单模特性.数值分析得到光子晶体光纤的基模的模式折射率,并由此研究了光子晶体光纤的波导色散与结构参量的关系.分析表明,光子晶体光纤的波导色散随空气孔孔距的变化符合Maxwell方程的比例性质.空气孔的相对孔径对波导色散有重要的影响.这些分析表明光子晶体光纤具有可以灵活设计其色散特性的潜在应用前景.     

Photonic Crystal Fibers: A New Class of Optical Waveguides

Remarkable properties of optical fibers with a high-index core region and a surrounding silica/air photonic crystal cladding have recently been reported. Here we discuss the physics, the special guiding properties, and the theoretical tools developed for the modeling of these photonic crystal fibers. With an emphasis on the applicational aspects of the fibers, we study their single-mode operation, bending losses, and dispersion properties. While exhibiting certain unique properties, the high-index core photonic crystal fibers share many common features with conventional optical fibers, attributed to an operation based on the well-known mechanism of total internal reflection. Fundamentally different from all high-index core fibers, in this work we demonstrate a novel type of optical waveguide, operating truly by the photonic bandgap effect. The novel fiber has an improved photonic crystal cladding and a central low-index structural defect along which the light is guided. The novel fiber has several unique features due to its different waveguidance mechanism, including remarkable dispersion properties and the potential to localize part of the guided mode in air regions. The results presented are fundamental in the field of photonic bandgap guidance, and this new class of optical waveguide is, therefore, expected to be of future interest to a large variety of research areas.     

Linearly Single Polarization Fibers with Zero Polarization Mode Dispersion

The optimum waveguide structure for linearly single polarization fibers, which satisfies the large modal birefringence and the zero polarization mode dispersion simultaneously, has been investigated. The basic waveguide structure is the single-mode optical fiber that has an elliptical core and stress-applying parts with a different expansion coefficient from that of the cladding. Waveguide parameters, such as index difference, core ellipticity, and cutoff wavelength, are first determined to obtain highly birefringent fibers with B = 1 X 10/sup -5/ or B = 5 X 10/sup -5/. The structure of the stress-applying parts that provides zero polarization mode dispersion is then determined.     

Optical spot-size converters for low-loss coupling between fibersand optoelectronic semiconductor devices

Structures of spot size converters that allow low loss and easy coupling between an optical semiconductor device and a fiber are proposed and designed theoretically. These spot-size converters have a tapered small core for expanding the mode field. They also have a double cladding region which consists of an n⁺-doped InP substrate as the outer cladding and a p-doped or nondoped InP layer as the inner cladding with a ridge structure. This double cladding utilizes the plasma effect of a carrier which makes the refractive index of highly doped n-InP lower than that of p-doped or nondoped InP. The double-cladding structure can tightly confine an expanded mode field in the inner cladding, and results in low radiation loss at the tapered waveguide, in addition, this structure reforms the mode field shape into a Gaussian-like shape and achieves a low loss coupling of less than 1 dB with a large misalignment tolerance for fiber coupling. These spot-size converters are easily fabricated and applicable to all types of optical semiconductor devices     

有双包层结构的楔变脊型光斑转换器的设计

为了提高硅基上单模波导器件与单模光纤的耦合效率和工艺容差,设计了一种实用新型光斑转换器,它由水平和垂直双向楔变的波导芯区和双内包层构成.分析了内包层折射率与其厚度对耦合效率的影响,发现通过优化内包层的参数可以加强对扩展光场的限制.针对在硅基上制作垂直楔变结构的波导还很困难的情况,提出了一种低成本、简单可行的制作方法     

Coupling of modes in bent biconically tapered single-mode fibers

Biconically tapered single-mode fibers are fabricated by heating a single-mode fiber while applying tension. As a result of this tapering, cladding modes are excited in the tapered region of the fiber where the V parameter of the fiber goes down below 1.0. These cladding modes couple to one another, leading to fluctuations of the optical throughput. Since the index difference between the cladding and the external medium is rather high (~0.5), the cladding modes will be of the exact type, namely, TE, TM, HE, and EH modes. The coupling of these modes in the tapered region is analyzed using the exact mode formalism when the tapered region is bent. The theoretical results agree very well with experimental results obtained on bent tapers which show strong fluctuations of the optical power as a function of the bending angle     

Periodic structures on biconically tapered optical fibers using ion beam milling and boron implantation

In this paper, the authors introduce local periodic perturbations of the cladding refractive index in the reduced diameter (waist) region of biconically tapered single-mode optical fibers by two methods. The first was focused ion beam milling to give short grooves perpendicular to the fiber axis, and the second was boron implantation, giving a phase grating in the cladding. Both structures exhibit a wavelength-dependent throughput and are highly sensitive to the environment in the waist region, which makes them potentially applicable in optical communication networks as well as in sensors. A simple corrugated slab-waveguide model is able to account for the measured spectral transmission of these structures.     

Thermal effects of Brillouin gain spectra in single-mode fibers

Brillouin gain spectra in two 250-m-long single-mode fibers with GeO₂-doped core/pure-silica cladding (fiber A) and pure-silica core/F-doped cladding (fiber B) were measured at temperatures ranging from -40 to +60°C at a wavelength of 1.32 μm. The temperature coefficients of Brillouin frequency shift were found to be 1.17 and 1.33 MHz/°C for fibers A and B, respectively. Temperature coefficients of Brillouin gain bandwidth were found to be -0.12 and -0.10 MHz/°C. These measurements provide useful information for applications of stimulated Brillouin scattering     

A novel hybrid coupler based on antiresonant reflecting opticalwaveguides

A novel hybrid coupler based on the antiresonant reflecting optical waveguides (ARROWs) is presented. This device consists of two parallel antiresonant reflecting optical waveguides with a tapered outermost cladding layer. Such a device can provide advantageous features of low radiation loss, low crosstalk at the output end, and compatible core dimensions and indexes with single-mode optical fibers. The staircase approximation and the eigenmode expansion analysis are used to analyze and optimize this device. In our design on a Si-substrate, radiation losses below 0.38 dB and extinction ratios below -20 dB for two waveguide channels are achieved. The beam propagation method (BPM) is also used to verify our design and analysis results     

Parametric study of depressed inner cladding single-mode fibers

A parametric study on the depressed inner cladding (DIC) single-mode fibers shows the influence of the inner-cladding diameter on the effective cutoff wavelengths of the first two modes, determining a practical transmission window. The set of curves that is obtained can be considered universal, at least for a value of the inner-cladding/core index differences ratio close to 0.5. The behavior of the effective cutoff wavelengths is explained by studying the effective index of the modes and the fundamental mode diameter, this explanation being confirmed by a bending and microbending sensitivity test. It appears that DIC fibers optimized for long-range transmission should have an inner cladding to core diameter ratio greater than 3 and a relative depth of the inner cladding smaller than 0.5.     

A simple laterally tapered waveguide for low-loss coupling tosingle-mode fibers

Low-loss coupling between semiconductor photonic devices and single-mode fibers is achieved using a simple InP/InGaAsP tapered waveguide. The proposed simple structure has a small and nearly square guiding core at its output facet. In this structure, the output field has a non-Gaussian profile, but low-pass filter coupling can be achieved by optimizing the design of the guiding core sizes. The waveguide is composed of a laterally tapered InGaAsP guiding layer and an InP cladding region on an InP substrate, facilitating integration of the waveguide with active devices using conventional processes. The waveguide is shown to have a total insertion loss of 2.6 dB, including a coupling loss of 0.9 dB and large ±2.5-μm misalignment tolerance in lateral and vertical directions with single-mode filters     

Guided Wave Analysis of Hollow Optical Fiber for Mode-Coupling Device Applications

In this paper, optical mode characteristics of hollow optical fibers are thoroughly analyzed using finite element method. The guided modes along the ring core and cladding are identified and their optical properties are investigated. For the core modes, we investigated intensity distribution, higher order mode cutoff, propagation constant, and chromatic dispersion. The mode coupling between the fundamental mode and the excited modes in both core modes and cladding modes are discussed for applications in mode-coupling devices.