Laser diode to monomode elliptic core fiber excitation via hemispherical lens on the fiber tip: Efficiency computation by ABCD matrix with consideration for allowable aperture

Employing Gaussian field distributions for both the source and the fiber, we report a simple theoretical prediction of the coupling efficiency of a laser diode to a single-mode elliptic core fiber via hemispherical lens on the fiber tip. The analysis is based on ABCD matrix for hemispherical lens under paraxial approximation. Our formalism takes care of the allowable aperture of the hemispherical lens. The present analysis prescribes analytical expressions for the coupling efficiency and the concerned calculations are executable by pocket calculator.     

卡塞格林望远镜耦合系统锥形导管的仿真与误差分析

为了提高卡塞格林望远镜像斑耦合多模光纤的效率,采用一种在光纤前端放置锥形导管的方法,利用光线在锥形导管中经过多次内全反射耦合进多模光纤中,通过几何光学理论及数值方法分析卡塞格林零视场光束像点的最大耦合角,确定锥形导管初始结构参数。将此参数作为Zemax仿真的初始条件,进行优化仿真计算和误差分析。结果表明:加工误差控制在0.03 mm之内,采用锥形导管后可以使像斑整体耦合效率较直接耦合提高4倍以上。     

Fiber coupling efficiency for random light and its applications to lidar

Employing the van Cittert-Zernike theorem of classical coherence theory, we derive a general expression for the efficiency with which quasi-monochromatic random light can be coupled to an optical fiber by means of a lens. For the important case of a source with Gaussian intensity distribution, we obtain and discuss the dependence of the coupling efficiency to single-mode fibers on the lens-to-fiber coupling geometry as well as on the ratio of lens size to speckle size. A specialization to the emerging fields of both incoherent and coherent fiber-based lidar applications shows that a maximum coupling efficiency of ~42% can be obtained for a monostatic system.     

The impact of the off-axis for the spatial light to multimode fiber coupling system

The electric field distribution of Gaussian beam and properties of Cassegrain optical antenna are discussed and their images have been simulated. By analysis of Cassegrain optical antenna with collimation and off-axis circumstance, laser power distribution and the curve of coupling efficiency are obtained. The question of spatial light to multimode fiber coupling efficiency with alignment and off-axis are researched. At last, taking into account off-axis situation of Cassegrain and laser beam to multimode fiber, the coupling efficiency of the whole system is discussed. These researches are necessary and useful for increasing spatial light to multimode fiber coupling efficiency.     

Estimation of coupling efficiency of optical fiber by far-field method

Coupling efficiency to a single-mode optical fiber can be estimated with the field amplitudes at far-field of an incident beam and optical fiber mode. We call it the calculation by far-field method (FFM) in this paper. The coupling efficiency by FFM is formulated including effects of optical aberrations, vignetting of the incident beam, and misalignments of the optical fiber such as defocus, lateral displacements, and angle deviation in arrangement of the fiber. As the results, it is shown the coupling efficiency is proportional to the central intensity of the focused spot, i.e., Strehl intensity of a virtual beam determined by the incident beam and mode of the optical fiber. Using the FFM, a typical optics in which a laser beam is coupled to an optical fiber with a lens of finite numerical aperture (NA) is analyzed for several cases of amplitude distributions of the incident light.     

激光束的锥台光纤传输

 分析了锥台光纤的传输特性,建立了高斯近似模型,采用模场耦合理论,计算了锥台光纤的功率转换效率。并在激光器的输出光波长为532 nm,多模光纤的数值孔径为0.11,纤芯半径为12.5 μm条件下对细端半径分别为（4±1）,（5±1）,（6±1）,（7±1）和（8±1） μm的锥台光纤的转换效率进行了实验测定。提出利用锥台光纤的圆柱形多模光纤部分传输光功率,锥台部分保证光束质量的传输方案,在保证光束质量的同时能传输较高的光功率。     

Zoned microstructure fiber for low-dispersion waveguiding and coupling to photonic crystals

We describe a zoned microstructure fiber that exhibits low dispersion and virtually zero spherical aberration because of its optimized piecewise Gaussian index profile. We present results of a nine-zone design that has an average refractive index of 2.3, a refractive-index contrast of 0.1, a first zone radius of 1.67 microm, and a maximum core radius of 5 microm. It has an in-fiber focal length of 8.88 microm and can focus light to a spot size of radius 315 nm, facilitating efficient coupling between single-mode fiber and photonic crystals.     

利用Kagome光纤实现多芯光子晶体光纤的输出合束

多芯光纤的输出光束只能在远场和焦点附近实现良好的同相位超模合束,这种超模传输特性大大影响了多芯光纤的应用范围.一种新型中空Kagome光纤为解决这一难题提供了可行的方案,利用中空Kagome光纤可以实现七芯光纤输出模式的整形合束.本文利用中心波长800 nm的钛宝石飞秒激光作为激光源,耦合入七芯非线性光子晶体光纤,得到700 nm至1050 nm的展宽光谱,并实现同相位超模输出.随后,将非线性展宽之后的宽谱七芯光束耦合至Kagome光子晶体光纤中,从Kagome光纤输出光斑呈高斯分布的模式传输,不再演变回七芯模式,耦合效率71%.实验还进一步验证此方法适用于不同结构的多芯光纤,为多芯光纤在高功率激光等领域的应用提供了参考.     

一种分析楔形光纤的等效矩形近似-阶梯串联法

楔形光纤(WSF)是实现平面光波光路芯片入出端口与光纤高效连接的核心部件. 采用数值模拟方法分析其中光波传输演化过程,是优化设计光子器件耦合结构的重要基础. 提出基于等效矩形近似的三维阶梯串联法(ERA-SCM),将楔端沿传输方向细分,引入矩形波导近似,给出了细分后各段近似矩形波导的等效折射率;在此基础上,建立了阶梯串联法分析模型,分级给出WSF中光波传输过程与模场演变. 数值分析结果表明,ERA-SCM比有限差分束传播法(FD-BPM)能够更精确地分析非对称光纤和波导结构,描述其中光模场的演化. WSF出射光场实测结果表明,ERA-SCM数值模拟结果与实验结果的误差为1.9%,而FD-BPM的误差为4.5%. ERA-SCM是分析非对称光波导光波传输与模场演变的有效方法. 关键词： 平面光波光路 楔形光纤 等效矩形近似 阶梯串联法     

锥台光纤的功率转换效率研究

 锥台光纤在激光束到单模光纤以及多模光纤到单模光纤的耦合中,在一些需要质量很高光束的同时有较高功率要求的场合有广泛应用。在这些应用中,锥台光纤的功率转换效率是很重要的一个参数。介绍了锥台光纤的熔拉研磨制作方法,分析了锥台光纤的传输特性。建立了高斯近似模型,采用模场耦合理论,计算了锥台光纤的功率转换效率。在以下参数条件下对细端半径分别为（4±1）μm,(5±1)μm,(6±1)μm,(7±1)μm和(8±1)μm的锥台光纤的功率转换效率进行了实验测定,激光器输出光波长532 nm;多模光纤NA＝0.11,纤芯半径a=12.5 μm;得到了与理论计算吻合的结果。     

Theoretical and experimental investigations on erbium-doped fiber amplifiers

A comprehensive model of erbium-doped fiber amplifiers (EDFA) is proposed in this paper, taking exited-state absorption (ESA), amplified spontaneous emission (ASE), background absorption, mode field, and Erbium ion radial distribution into account. Computer programs capable of handling forward, backward, and different wavelength bidirectional pumping are developed. Among other things, it is found that under typical conditions, Gaussian field approximations may cause considerable errors. The optimum V-value for small signal gain is much smaller than that for minimum Gaussian or Desurvire spot size; ASE propagating opposite to the pimp beam can be several times higher than at propagating along the pump beam, resulting in a faster-than-linear decrease of the pump power at its beginning portion. Experimental results of domestic-made erbium-doped fibers pumped by 980 nm and 1480 nm laser diodes are also given.     

固体激光器与光纤激光器对光子晶体光纤棒耦合的分析与对比

耦合效率的高低与耦合后光斑的好坏直接影响着光子晶体光纤棒的放大效果,因此需要对种子光的耦合效果进行研究,选择合适的激光器作为种子源。本文对光子晶体光纤棒在固体激光器与光纤激光器两种情况下的耦合效率进行了理论分析;模拟计算了两种激光器情况下耦合效率的变化规律以及对准误差对耦合效率的影响;选择合适的透镜或透镜组,使用两种激光器对光子晶体光纤棒进行了耦合实验;对比两种激光器的耦合效果可知:固体激光器的耦合效率最高只能达到62.4%,而光纤激光器的耦合效率可以达到80%以上;在光纤激光器耦合情况下,对不同功率注入时耦合效率,以及耦合后光斑进行了实验分析。得到的实验结果对后续光纤棒的放大实验具有指导作用。     

ABCD matrix model of quadric interface-lensed fiber and its application in coupling efficiency calculation

In order to simplify the theoretical coupling efficiency calculation of a laser diode (LD) to a single-mode fiber (SMF) with a quadric lens formed on the fiber tip, based on the Gaussian beam approximation, using the ABCD matrix method the relationship between the coupling efficiency and the structure parameters of quadric surface-lensed fiber is studied and analyzed in detail, and the coupling efficiency can reach as high as 99.87% with optimizing parameters of lensed fiber in theoretical simulation. Meanwhile, under paraxial approximation, the ABCD matrix for refraction of different quadric surface micro-lens can be formulated as a unified expression. This result can give some guidance suggestions for fabricating the quadric interface-lensed fiber.     

光固化成型中紫外光源-光纤耦合研究

CPS(Compact Prototyping System)光固化成型系统中,以紫外灯作为光源,通过光纤传输紫外光到液面,进行扫描成型.光源通过椭球反射镜集光,在焦点处与光纤耦合,提高了光源-光纤的耦合效率,在实际的光源-光纤耦合中,光纤的装配存在着角度误差、离焦误差、离轴误差三种形式,这些误差影响传输光束的传输效率.在光纤的输出端,聚焦透镜将紫外光汇聚,光斑尺寸更小,能量更加集中.CPS成型机中使用的紫外光的能量并不是均匀分布的,光斑的中心能量高,边缘能量低,不同的能量分布影响树脂固化的形貌和质量.     

A graded-index fiber taper design for laser diode to single-mode fiber coupling

A tapered graded-index fiber (GIF) structure is, for the first time, proposed for the efficient fiber-laser coupling. A simple analytical approach based on the ABCD law is utilized to optimally design the parameters of the hemispherically-ended GIF taper. A wide-angle beam propagation method is then employed to compute the fiber mode fields transmitted through the fiber taper in order to evaluate the coupling efficiency. By taking one commercially available GIF as an example, the effectiveness of the new design reported here is verified with a demonstration of high coupling efficiency of nearly 90%.     

Detection of non-paraxial optical fields by optical fiber tip probes

The optical response of a single-mode uncoated fiber tip to a 3D polarized field, including longitudinal components, is investigated. The 3D field is produced by an opportune superposition of TE and TM plane waves. The contribution of the different field components to the detected signals was discriminated by integrating the scanning probe microscope with a multi-heterodyne detection technique. A simple coupling model for the tip is introduced. The longitudinal field component was assumed to couple to the transverse fiber modes through complex coupling coefficients. Coupling coefficients were obtained by fitting the parameters of the model to the experimental data. These results demonstrate that the longitudinal components of the field are coupled by this probe with an efficiency approximately equal to that of the transverse polarization components.     

楔形光纤与半导体多量子阱平面光波光路芯片的耦合分析

采用楔形光纤(WSF)实现了与半导体多量子阱(MQW)平面光波光路(PLC)芯片的高效耦合。在多量子阱-平面光波光路前置模斑转换器(SSC)和不加模斑转换器的情况下,用阶梯串联法(SCM)数值模拟并优化设计了楔形光纤-平面光波光路间最佳耦合参量:楔形光纤楔角45°、端面圆柱透镜曲率半径2.5μm、模斑转换器-多量子阱-平面光波光路出射椭圆光斑长半轴3.5μm、纵横比5、楔形光纤-平面光波光路间垂直方向和水平方向无偏移、纵向间距5.5μm。用反向推演法(IDM)实验分析了楔形光纤样品的出射光场,与阶梯串联法(SCM)计算结果相比长轴误差为3.125%,短轴误差为0.8%。建立楔形光纤-平面光波光路-单模光纤(SMF)的耦合实验系统,在1.55μm波长处以单模光纤作为出纤的相同条件下,发现楔形光纤激励入射平面光波光路比单模光纤和锥形透镜光纤(TLF)作为入纤的耦合效率分别提高了24.827 dB和16.22 dB,为多量子阱-平面光波光路芯片尾纤封装技术提供了实验原型。     

基于光纤组束激光的激光干扰技术研究

为了增强激光干扰技术的机动性和灵活性,迫切需要体积小、重量轻的高功率激光干扰源。通过理论分析和数值计算,结果表明,光纤组束激光具有接近80%的组束耦合效率,并可以达到10kW量级的输出功率。以硅材料CCD为例,该功率量级的组束激光可以有效地实施对光电探测器的饱和干扰以及由温升导致的硬损伤。光纤组束激光是一种新型的激光干扰源,在机载光电对抗领域具有广泛的应用前景。     

基于光束填充的多单管半导体激光器光纤耦合

为了进一步提高多单管半导体激光器的输出功率，通过对常见的阶梯型多单管半导体阵列进行分析，提出在光斑尺寸较小的慢轴方向对光束进行填充，在同样的耦合条件下，使更多的激光能量耦合进光纤中，实现更高功率的输出。文中使用光参数积作为评价光束质量的指标，论证了慢轴光束填充的可行性，利用ZEMAX仿真软件对8路常见阶梯型多单管半导体阵列和12路填充阵列进行对比仿真，在不影响耦合效率的前提下，实现了将12路波长为860 nm、输出功率3 W的单管半导体激光器耦合进芯径105 μm、数值孔径0.22的光纤中，光纤输出功率为33.4 W，光纤耦合效率为92.78%。仿真结果表明，对慢轴方向进行光束填充可以在一定程度上提高多单管半导体激光的功率输出。