A review of biconical taper couplers

Abstract

An optical coupler distributes light from a main fiber to one or more branch fibers. Optical signals can also be passed bidirectionally along a single fiber [1]. Usually couplers are passive devices which are attached to other components in a system by means of optical fiber connectors, or may be joined to them by splicing. Several basic coupler designs have been discussed in the literature, each having some advantages and disadvantages. The biconical taper coupler [2] consists of two fibers that are fused and subsequently tapered. These fibers may have step or graded index profiles. Beam-splitter types [3] (consisting of discrete components) are generally expensive. Mixing rods are simple and can be ruggedly constructed, but an extra 3dB loss above packing fraction loss is incurred when used with graded index fibers [4].     

全光纤反射式马赫曾德尔干涉型多波长滤波器的研究

对反射式的马赫－曾德尔干涉型多波长滤波器的性能作了较详细的研究，得出了两个耦合器的分光比和两臂的臂长差对其输出谱线的影响，结论对设计制作不同性能指标的该型多波长滤波器具有重要的参考价值。实验上用光纤熔融拉锥法制作了该种器件，实验结果与理论相符合。     

High efficient Tm3+ doped double cladding silica fiber laser with an intracavity biconical taper

A high efficient LD (laser diode) pumped Tm³⁺ doped double clad silica fiber laser with an intravacity biconical taper was reported. A biconical taper located ～3 cm from the output end of the fiber laser was fabricated by heating and stretching method with a length 1.5 cm and waist diameter ～20 µm. The slope efficiency was 49.8% with respected to the launched pump power, and the maximum output power was 1.97 W. Pre and post output laser power ratio was ～10. This fiber laser was compared with other three biconical tapered fiber lasers (the same fiber with different tapers) and a uniform geometry fiber laser. With intracavity biconical tapers, fiber lasers’ thresholds were ～1 W higher than the fiber laser without the taper (1.97 W). The pump end’s slope efficiencies of fiber lasers with tapers were 3–5% in contrast with 37.6% of the uniform one. After tapered, the pre and post laser power ratios were much higher than the un-tapered one’s, but not changed much with the launched pump power.     

A review of biconical taper couplers

An optical coupler distributes light from a main fiber to one or more branch fibers. Optical signals can also be passed bidirectionally along a single fiber [1]. Usually couplers are passive devices which are attached to other components in a system by means of optical fiber connectors, or may be joined to them by splicing. Several basic coupler designs have been discussed in the literature, each having some advantages and disadvantages. The biconical taper coupler [2] consists of two fibers that are fused and subsequently tapered. These fibers may have step or graded index profiles. Beam-splitter types [3] (consisting of discrete components) are generally expensive. Mixing rods are simple and can be ruggedly constructed, but an extra 3dB loss above packing fraction loss is incurred when used with graded index fibers [4].     

单模光纤双圆锥体的传输特性

本文研究并制作了光纤双圆锥体滤波器,给出了耦合模总相位差与滤波性能之间的关系,讨论了推的对称性对滤波器峰值传输的影响,对不同结构的锥体谱特性进行数值计算以验证分析结果,最后给出了实验结果对照.     

Fused biconical tapered fiber-optic devices: Application to data buses

Abstract

The technology of fiber-optic couplers based on fused biconical tapered structures is reviewed, and the linear and star data-bus configurations that can be constructed using these couplers are discussed. A useful network topology for interconnecting star couplers using the hybrid transmission-reflection star coupler is also proposed. This network configuration has the advantage of requiring significantly less optical fiber than networks employing a single transmission or reflection star.     

Study of all-fiber flat-top passband interleaver based on 2 × 2 and 3 × 3 fiber couplers

An all-fiber interleaver of cascaded Mach-Zehnder interferometer (CMZI) using a 2 × 2 and three 3 × 3 fiber couplers with almost rectangular wavelength response is developed by using a new configuration. Based on the digital approximation method of Fourier series, the relationships among the coupling ratios of the couplers, the length difference of interference arms and the intensity transmission spectrum are analyzed. The results indicate that a uniform flat-top spectral response can be obtained by the configuration when the length difference of interference arms and the coupling coefficient of the couplers, with some certain values, are given. The passband and stopband of optical interleaver are improved remarkably. Compared with conventional CMZI using 2 × 2 fiber couplers, the most advantage of the present method is that the coupling ratio of couplers can be controlled accurately because the interference effect of the monitor light can be sheered away in the course of fabricating couplers. A novel structure of filter is fabricated by using the fused biconical taper technics in experiment. The experiment results are in good agreement with the analytical ones.     

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.     

Influence of tapering process on changes of optical fiber refractive index distribution along a structure

Investigations of internal structure changes along an optical fiber during the manufacture of biconical taper have been described. Basing on the constant volume theory, classification of biconical structures manufactured on a special set-up is presented and discussed. The interferometric tomography method has been used for determination of 3D geometry and refractive index distribution in manufactured optical fiber tapers. The experiments provide detailed information on external (diameter of cladding) as well as internal (core diameter and refractive index profile) changes along the taper region. The results have been discussed in relation to the parameters of the manufacturing process.     

Cavity ringdown strain gauge

Biconical tapered single-mode fiber, which is common in many telecommunications components, offers an alternative sensor to typical optical fiber strain gauges that are susceptible to temperature and pressure effects and require expensive and sophisticated signal acquisition systems. Cavity ringdown spectroscopy, a technique commonly applied to high-sensitivity chemical analysis, offers detection sensitivity advantages that can be used to improve strain measurement with biconical tapers. Combining these two technologies in a spatially extended resonator, we demonstrate a minimum detectable change in ringdown time of 0.08%, corresponding to a minimum detectable displacement of 4.8 nm, and a sensitivity to strain as small as 79 n epsilon/square root(Hz) over a 5-mm taper length.     

利用改进的熔融拉锥法制备单模光纤波分复用器

研究了熔融拉锥光纤型波分复用器制备过程中各相关参量对其最小信道间隔的影响。并根据理论模拟的结论设计了新型的熔融拉锥装置,其中加热装置为有较长均匀加热区域的条形加热炉,使熔融拉锥光纤型波分复用器的耦合长度显著加长,改善了传统拉锥方法对这一类型波分复用器可达到的最小信道间隔的限制,得到了初步实验结果,最小信道间隔达到3.01 nm。     

Fused biconical tapered fiber-optic devices: Application to data buses

The technology of fiber-optic couplers based on fused biconical tapered structures is reviewed, and the linear and star data-bus configurations that can be constructed using these couplers are discussed. A useful network topology for interconnecting star couplers using the hybrid transmission-reflection star coupler is also proposed. This network configuration has the advantage of requiring significantly less optical fiber than networks employing a single transmission or reflection star.     

1.5μm波导声光可调滤光器研究

对１．５μｍ波导型声光ＴＥ－ＴＭ模式转换光滤波器进行了研究。采用了可降低驱动功率的表面声波导结构。在１．５￣１．６μｍ波段实现了可调滤光，通带宽度小于２ｎｍ，达到９７￥以上转换率所需的射频驱动功率约８０ｍＷ（１９ｄＢｍ），器件总插入损耗约９ｄＢ。     

Small-scale integration of fused tapers

The Small-Scale Integration (SSI) of fused couplers is proposed. The idea is to make several couplers on a single continuous piece of fiber rather than making discrete couplers and then splicing them together. Rudimentary tooling was set up to fabricate the couplers. Nine devices were successfully integrated onto a single fiber. In addition, a theory of fused tapers was developed. The fabrication parameters are analytically related to fiber geometry, which in turn, is related to the optical performance of the device.     

Characteristics of Fused Optical Fiber Grating Couplers with Tapered Shapes

Fused optical fiber grating couplers (FGCs), in which Bragg gratings are written in the coupling regions of fused optical fiber couplers with tapered shapes along them are theoretically and experimentally investigated. By coupled mode analysis considering these tapered shapes, the drop efficiencies of the FGCs, which are inherently zero for the normal fiber couplers without grating, are calculated. The grating offset, which is the difference between the centers of grating and the coupler taper, is found to be effective to increase the drop efficiency of the FGC. It is determined that the tapered shapes influence the wavelength dependence of the drop efficiency and tolerate the grating offset precision. Apodizations for the grating of FGC are found to be effective in suppressing side lobes of the drop efficiency. Measurement of the wavelength characteristics of drop efficiency and the temperature characteristics of drop wavelength of the fabricated fused FGC confirm that the lightwave with specific wavelength can be dropped by the FGC and that the temperature dependence of the drop wavelength is similar to that of Bragg wavelength changes of usual fiber Bragg gratings.     

锥形光纤中光波传输特性的蒙特卡罗模拟

为了分析锥形光纤中光子流的传输特性和空间分布特性,采用了蒙特卡罗算法对其进行了模拟仿真,得到了大量光子经过锥形光纤后的光子分布图.模拟实验结果表明,通过锥形光纤的大量光子集中在探测器中心,到达探测器某一位置的光子数随着该位置到探测器中心距离的增加而减少,并且在锥形光纤锥度比一定的条件下,改变锥形光纤长度,光强度随着长度的增加而下降,但通过锥形光纤的光子分布是一样的.这些研究结果对锥形光纤的设计和实际应用提供了重要依据.     

塑料光纤双锥与半球腔的活动耦合效应

报道了用氮氖激光实现的塑料光纤双锥与介质半球(称半球腔)之间的活动耦合效应。测量了光纤双锥的发射效率，并测量了锥腔耦合效率与半球腔的表面积、锥腔距离之间的关系。根据这种效应，可以制成塑料光纤活动耦合器；如果再加上调谐滤波器，可制成适合塑料光纤通信网用的信号上下载装置。     

Photonic crystal fiber coupler

Fiber couplers made with photonic crystal fibers (PCF) are reported. Two types of PCF were fabricated by means of stacking a group of silica tubes around a silica rod and drawing them. The fiber couplers were made by use of the fused biconical tapered method. With a fiber that had five hexagonally stacked layers of air holes, a 33/67 coupling ratio was obtained, and with a one-layer four-hole fiber, a 48/52 coupling ratio was obtained. The fabrication processes and the characteristics of the PCFs and the PCF couplers are presented.     

The integrated prism interpretation of multileg Mach-Zehnder interferometers based on multimode interference couplers

To simplify the driving conditions as well as to improve the wavelength response of multileg Mach-Zehnder interferometers (MZI) based on multimode interference (MMI) couplers we develop a new interpretation of these devices. Used for optical switching and for wavelength filtering, they are shown to work as active or passive integrated prisms. This analogy greatly facilitates the design of such components and simplifies the understanding of their functionality. The driving conditions of optical switches based on such multileg MZI are easility studied and will be reduced to basic active phase shifts. For wavelength filtering applications using such elements, a new design of the phased waveguide array is proposed which improves the characteristics of the filters.     

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%.