Surface corrugation Bragg gratings on optical fiber tapers created via plasma etch postprocessing

We describe a reflection-based fiber filter fabricated by plasma etching a surface corrugation Bragg grating on a tapered single-mode fiber. The taper waist with the grating forms the functional part of the filter, and the adiabatic taper transition removes unwanted higher-order modes. The spectral response is controlled by varying the taper diameter while maintaining a constant grating period. Reflection spectra have been investigated theoretically and experimentally and found to be in good agreement.     

A long uniform taper applied to an all-fiber Tm<Superscript>3+</Superscript> doped double-clad fiber laser

A long uniform taper fabricated on a large mode area Tm³⁺-doped double-clad fiber laser, which was clad-pumped by a laser diode (LD), was found to be an effective wavelength filter while improving the output beam quality and narrowing the line-width significantly. The long uniform taper was fabricated directly on the multi-mode Tm³⁺-doped fiber by heating and stretching method, and located several centimeters before the output fiber end. By slightly bending the taper section, the output laser spectrum was left with only one peak with a line-width less than 0.5 nm, compared to the multi-peak spectrum with a 5 nm line-width before tapering, indicating that the multi-mode fiber could produce quasi-single wavelength output with a long uniform taper. The beam quality factor M ² declined from 6.6 to 2.6 compared. Only a slight decrease in slope efficiency, from 19.2 to 17.5%, was observed. The main output wavelength had a blue shift of 8 nm.     

Small-core single-mode microstructured polymer optical fiber with large external diameter

A preform sleeving technique is demonstrated that allows the fabrication of single-mode polymer microstructured fiber with the smallest core and hole dimensions yet reported to our knowledge. For a fixed triangular hole pattern a range of fibers is produced by adjustment to the operating conditions of the draw tower. Numerical modeling is carried out for one of the fibers produced with a 570-microm external diameter, a core diameter of 2.23 microm, an average hole diameter of 0.53 microm, and an average hole spacing of 1.38 microm. This fiber was shown to be endlessly single mode.     

双层孔大模场微结构光纤的模式特性分析

本文分析一种由两层低折射率孔组成的大模场光纤的模式特性.采用多极法和有限元方法数值计算并分析了内层孔与外层孔尺寸变化对光纤基模与高阶模的束缚损耗与弯曲损耗的影响,设计获得了一种具有较高的高阶模和基模损耗比同时允许一定程度弯曲的大模场光纤.结果表明:当内层孔直径为34 μm,外层孔直径为24 μm时,其基模束缚损耗为0.000 17 dB/m,而高阶模束缚损耗为1.39 dB/m;光纤的基模模场面积为2 150.9 μm²,当弯曲半径为1.2 m时,弯曲损耗为0.106 dB/m.     

Passive fiber-optic components made by the fused biconical taper process

The rapid development and deployment of optical fiber for a variety of applications has resulted in a similarly rapid development of myriad components to effectively utilize the fiber. Thus, for example, significant work has been done over the past several years in the development of sources and detectors, connectors, and splices. These components share the common trait of being external to the fiber and hence not part of the fiber per se. Another class of components, however, is made from the fiber itself. A number of components, such as polarizers, analyzers, and mode or wavelength filters, require only simple bending of the fiber in a specified manner. On the other hand, such techniques as polishing, etching, and heating can also be used to alter the transmission properties of the fiber.

While a number of interesting devices have been made by these procedures, the present paper is limited to devices made by the biconical taper process. If a fiber is heated and stretched, a biconical taper is formed, which can significantly alter the transmission characteristics of the fiber. Interesting wavelength filters and sensors can be made with single tapers or combinations of tapers. Moreover, if two or more tapered fibers are fused, then coupling of light from one fiber to another can be achieved. The tapering and fusing processes can be used to make wavelength-enhanced couplers, which can be used as wavelength-division multiplexers or nearly wavelength-independent couplers or splitters.

In this paper, we discuss the components made by the biconical taper process, their fabrication, in both theory and practice, and their performance. Practical implementations of these components are also discussed, including their manufacturability and reliability.     

Influence of a holey cladding structure on spectral characteristics of side-polished endlessly single-mode photonic crystal fibers

We experimentally investigate the spectral characteristics of side-polished endlessly single-mode photonic crystal fibers of different polishing depths and radii of curvature that are important to mode field diameter, evanescent coupling, waveguide losses, higher-order mode excitation, and dispersion slopes of photonic crystal fiber components. A polished photonic crystal fiber with a greater polishing depth or a larger radius of curvature shows a more dispersive characteristic. In contrast to conventional single-mode fibers, in photonic crystal fibers the evanescent field is more strongly localized, and the propagating light can be more efficiently guided within the deformed core.     

Designing of endlessly single mode polarization maintaining highly birefringent nonlinear micro-structure fiber at telecommunication window by FV-FEM

An endlessly single mode highly polarization maintaining nonlinear microstructure fiber at telecommunication window is reported via full-vector finite element method. By taking three ring hexagonal PCF with suitable fiber parameter such as air hole diameter in cladding region d = 0.8 μm, pitch 2.3 μm and introducing four symmetrical large air holes near core region d′ = 2 μm, single mode (V_eff ≤ π), small effective mode area 2.7 μm², nonlinear co-efficient 44.39 W⁻¹ km⁻¹, high phase birefringence of the order of 10⁻³ and group birefringence of the order of 10⁻⁴ with beat length 0.3 μm at wavelength 1.55 μm are achieved.     

Switchable Multi-Wavelength Erbium-Doped Fiber Lasers based on a Mach-Zehnder Interferometer Using a Twin-Core Fiber

A switchable multi-wavelength erbium-doped fiber ring laser based on a compact in-fiber Mach-Zehnder interferometer comb filter at room temperature is presented. The comb filter is formed by splicing a section of twin-core fiber between two single mode fibers. By adjusting the states of the polarization controller appropriately, the laser can be made to operate in stable single-, dual- and three-wavelength lasing states. The operation principle is based on spectral hole burning induced by the saturated effect and polarization hole burning.     

Wideband frequency tunable optoelectronic oscillator based on a dispersion compensated microwave photonic filter

A wideband tunable optoelectronic oscillator (OEO) based on a dispersion compensated phase modulated microwave photonics filter (MPF) is proposed and experimentally demonstrated. The MPF, consisting of a tunable laser source (TLS), a phase modulator, a fiber Bragg grating Fabry–Perot filter and a photodetector, is used as the oscillating mode selection device. Dual-loop configuration is employed to make sure the OEO operates at a single oscillating mode. Theoretically analysis is carried out to demonstrate the tuning range limitation due to the phase-modulation to intensity-modulation conversion caused by long single mode fiber (SMF). To overcome this disadvantage and expand the signal tuning range, a dispersion compensation fiber is introduced followed by the SMF to compensate the fiber dispersion. By simply adjusting the wavelength of the TLS, an ultra-wideband tunable microwave signal with the frequency tuning range from 2 to 44 GHz is obtained. The phase noise is also investigated and measured to be less than ? 110 dBc/Hz at an offset of 10 kHz within the whole frequency range.     

Improved large-mode-area endlessly single-mode photonic crystal fibers

We numerically study the possibilities for improved large-mode-area endlessly single-mode photonic crystal fibers for use in high-power delivery applications. By carefully choosing the optimal hole diameter, we find that a triangular core formed by three missing neighboring air holes considerably improves the mode area and loss properties compared with the case with a core formed by one missing air hole. In a realized fiber we demonstrate an enhancement of the mode area by approximately 30% without a corresponding increase in the attenuation.     

Fusion splicing small-core photonic crystal fibers and single-mode fibers by controlled air hole collapse

A method based on controlled air hole collapse for low-loss fusion splicing small-core photonic crystal fibers (PCFs) and single-mode fibers (SMFs) was demonstrated. A taper rig was used to control air hole collapse accurately to enlarge the MFDs of PCFs which was then spliced with SMFs using a fusion splicer. An optimum mode field match at the interface of PCF-SMF was achieved and a low-loss with 0.64 dB was obtained from 3.57 dB for a PCF with 4 μm MFD and a SMF with 10.4 μm MFD experimentally.     

纤维光锥有效透过率的理论分析

光锥的有效透过率不仅影响光锥与CCD的耦合效率,而且会影响到耦合器件的信噪比.本文主要从理论上讨论了影响光锥有效透过率性能的因素,从纤维光学元件的实际内部结构推导出了其实际有效填充率,从光线在光锥中的传输特征推导出了光线在光锥中传播时产生的衰减损耗,最后提出了提高光锥有效透过率的方法和途径.     

2×2熔锥型单模光纤耦合器的模型

选择适当的连续函数描述了熔锥型耦合器熔锥形状的渐变特性及其相互位置关系,然后分别应用局部模式理论和变分法分析了横截面沿纵向变化的锥形区域和横截面近似不变的耦合区域内的耦合行为,得出了耦合器中任一横截面处的耦合系数和耦合功率表达式计算表明,熔锥型耦合器的总功率耦合主要发生在耦合区域,在两锥形区域,对于纤芯归一化频率小于1.08的部分,也发生了相当程度的耦合为了使耦合器的理论模型更接近于实际物理情况,锥形区域的耦合作用也是不容忽略的,从而得到了一个更精细的熔锥型光纤耦合器的模型.     

Refractive index measurement based on core-cladding mode interferometry in endlessly single mode fiber

We propose a simple almost “all-fiber” sensor based on intermodal interference of core and cladding mode in an endlessly single mode photonic crystal fiber. Sensitivity to an external refractive index is studied experimentally and theoretically.     

Beam quality factor of mixed modes emerging from a multimode step-index fiber

The beam quality factors (or M² parameter) of coherent and incoherent superposition of the several lower-order LP modes emerging from a step-index fiber have been calculated by using the second-order moment method. The results indicate that, for an individual LP mode, the M² parameter takes its maximum value when the normalized frequency V of the step-index fiber approaches the cut-off frequency, and it gradually becomes constant as V increases. In the case of incoherent superposition, the larger the fraction intensity carried by the higher-order mode, the larger the beam quality factor M². Under certain circumstances, the value of the M² parameter of the mixed mode that comprises several LP-modes contents may become even smaller and closer to the ideal Gaussian beam than that of the fundamental mode in a step-index fiber. However, in the case of the coherent superposition, the value of the M² parameter of the mixed mode may be greater than that of the higher-order constituent mode. The results reported here could be helpful for the application of the high-power fiber laser systems.     

电光聚合物波导中的锥形结构

提出并设计了一种基于电光聚合物的锥形波导,可用于单模光纤与电光聚合物波导器件之间的连接.锥形波导中采用了宽度锥形和折射率锥形结构.宽度锥形采用劈形形状,通过宽度和折射率的缓慢变化实现模场转换.劈形形状的宽度锥形具有较小的损耗且易于制作,折射率锥形可采用灰度掩膜光刻技术制作.研究了锥形波导的传输损耗与锥形波导的长度、波导宽度和厚度、材料吸收损耗等参数的关系及其优化,分析了锥形波导中的功率传输、模场分布与模式转换效率.结果显示锥形波导的传输损耗小于0.37 dB,光纤-波导-光纤的连接损耗优于1.62 dB,对插入损耗的改善达到8.78 dB,模场转换效率达到了83.7%.     

Optimization of core size in erbium doped holey fiber amplifiers

Holey fibers (HFs) can be used as amplifier in optical communication systems. These new fiber structures have flexibility to change the fiber parameters such as refractive index of guided mode, air filling factor (AFF), V number, group velocity dispersion (GVD) and numerical aperture, only by the change of the hole size and air-hole spacing. In this paper we will use improved fully vectorial effective index method (IFEIM) to analyze the erbium doped holey fiber amplifier (EDHFA) with hexagonal unit cell. The range that core size can change in any air-hole spacing will determine and the growth of core size in the amplifier character such as maximum gain and optimum length will study. By determination of single mode region the effect of pump power and dopant concentration growth, on the amplifier parameter will define.     

基于光纤拉锥模场匹配技术的光子晶体光纤低损耗熔接

光子晶体光纤(PCF)与普通单模光纤(SMF)的低损耗熔接技术是光子晶体光纤走向实用化必须解决的关键技术问题。提出一种基于光纤拉锥模场匹配技术的光子晶体光纤低损耗熔接的新方法,利用熔融拉锥机对模场不匹配的两类PCF进行预处理,结合常规电弧放电熔接技术对三种不同类型的PCF与SMF的熔接损耗进行实验研究。在实验中通过精确控制熔融拉锥机各种参数,实现了不同模场直径PCF和SMF的模场匹配,该方案同时通过优化各种电弧放电参数,使熔接后的损耗降到了0.3dB以下,实现了PCF-SMF之间低损耗、高强度的熔接,满足了不同模场PCF实际应用的要求,具有很好的潜在应用价值。     

Leakage of the fundamental mode in photonic crystal fiber tapers

We report detailed measurements of the optical properties of tapered photonic crystal fibers (PCFs). We observe a striking long-wavelength loss as the fiber diameter is reduced, despite the minimal airhole collapse along the taper. We associate this loss with a transition of the fundamental core mode as the fiber dimensions contract: At wavelengths shorter than this transition wavelength, the core mode is strongly confined in the fiber microstructure, whereas at longer wavelengths the mode expands beyond the microstructure and couples out to higher-order modes. These experimental results are discussed in the context of the so-called fundamental mode cutoff described by Kuhlmey et al. [Opt. Express 10, 1285 (2002)], which apply to PCFs with a finite microstructure.     

A stable and tunable linear polarization single longitudinal mode fiber ring laser

A stable and tunable linear polarization single longitudinal mode fiber ring laser is proposed and demonstrated. A high extinction ratio spatial mode interference filter utilizing two-mode graded-index fiber supresses irregular side modes effectively, the combination of fiber polarizer and polarization maintaining fiber act as a polarization dependent comb filter, and tunable bandpass filter is adopted to select proper lasing wavelength. The power fluctuation and wavelength fluctuation at 1549.88 nm are less than 0.05 dB and 0.02 nm in 1 h, respectively. Only one longitudinal mode appears within a free spectral range of 7.5 GHz, and the degree of polarization in percentage is approximately as high as 99.07%.