Nd:YAG laser delivery system utilizing GRIN rod lenses as focusing-collimating optics

A sequential electromechanical system capable of delivering the radiation coming from a medium power Nd:YAG laser to an array of multimode optical fibres is presented. The system utilizes GRIN-rod microlenses as focusing-collimating optics. A focusing unit positioned at the fibre distal-end, which can work in two different configurations, has also been designed and constructed.     

Optical fibers with low nonlinearity and low polarization-mode dispersion for terabit communications

Refractive-index nonlinearities have negligible effect on the performance of short-haul fiber-optic communication links utilizing electronic repeaters. However, in long links, nonlinearities can cause severe signal degradations. To mitigate nonlinear effects, a new generation of fibers, referred to as large effective-area fibers, have been introduced in recent years. This paper reviews the latest research and development work on these fibers conducted by several research groups around the world. Attention is focused on a class of large effective-area fibers that are based on a depressed-core multiple-cladding design. Another important issue in long-haul and high capacity fiber optic systems is the polarization-mode dispersion (PMD) which has been recognized as a serious limiting factor. In this paper, an improved fiber design is proposed which, in addition to providing large effective-area and low bending loss, eliminates PMD due to elliptical deformation in the single-mode wavelength region. Furthermore, this design is allowed to provide a small chromatic dispersion about few ps/nmkm, in order to overcome four-wave mixing effects.     

Bit-rate adaptive optical performance monitoring method for fiber communication systems

A novel bit-rate adaptive method, by varying the optical sampling rates alternatively, is proposed in this paper for optical performance monitoring. Firstly, the theoretical model and the differential software-synchronized algorithm are developed. Then, the results verify that different channel bit-rate can be estimated with high precision irrespective of the modulation formats and signal distortion caused by chromatic dispersion and nonlinearity along the fiber link. Employing the proposed bit-rate adaptive method, the eye diagrams and Q values of 10 Gbit/s, 40 Gbit/s and even higher bit-rate signal can be monitored by a single optical performance monitoring system without any prior knowledge about bit-rate or signal period. The method we propose in this paper has the advantage that different channel bit-rates can be adaptively estimated and the differential software-synchronized algorithm is much simpler.     

Influence of surface errors on the performance of EFPI based on GRIN lenses

The graded index (GRIN) lens reduces the divergence angle of the beam exiting from the lead-in optical fiber, which results in the increasing of F–P cavity mirror areas. Surface errors of the increased mirrors cannot be ignored due to the machining tolerance. The influence of surface errors on the performance of the fiber extrinsic Fabry–Perot interferometer (EFPI) is researched. Theoretical analysis and numerical simulation results demonstrate that surface errors will diminish the interference contrast. The larger the surface error, the smaller the interference contrast will become. The results will contribute to the design of an EFPI with a certain interference contrast.     

GRIN透镜干涉仪光学系统模型和分析

较完整地提出GRIN透镜干涉仪的数学模型,分析了光学系统处于六种不同模态下的特点,讨论了有关结构因素,特别是GRIN透镜长度对干涉仪工作性能的影响,有关分析和相关结论可作为GRIN透镜干涉仪系统设计和测量计算的依据。     

Theoretical model for the transverse interference pattern of GRIN optical fiber using a laser sheet of light

A new technique (El-Ghandoor et al., Opt. Laser Technol. 31(7) (1999) 481–488) has been applied to study the shape of transverse interference fringes, instead of multiple beam Fizeau fringes (Marhic, Stein, Appl. Phys. Lett. 35 (1975) 1678–1682), from a GRIN optical fiber. In this technique, a laser light sheet is used to illuminate and pass directly through the optical fiber. Theoretical expressions are derived for the optical path differences of three groups of interference beams. The first one passes through the cladding, the core, and then the cladding once again; the second passes through the cladding only, and the third passes through the surrounding air. Theoretical expressions for the shape of transverse interference fringes formed across isotropic non-absorbing optical fibers are also calculated.     

光纤制导技术及其应用

介绍了光纤制导系统的组成；分析了导弹与控制站之间数据传输、转换、处理和制导原理；采用单模双通道波分复用技术和时分复用技术，通过对两种不同波长光的耦合、分离，实现了单根光纤双向传输，给出了光纤制导导弹中的技术关键和要求，包括制导光纤、图像传感器、双向通信传输技术、光源器件和光检测器件等．     

Power penalty of cascaded Bragg gratings in the presence of PMD and PDL

Considering the presence of birefringence in Bragg gratings, the spectral polarization mode dispersion (PMD) and polarization dependent loss (PDL) induced power penalty of eye opening is examined for a network with cascaded Bragg gratings. It is shown that when the birefringence in the grating is less than 10⁻⁵, the induced power penalty is less than 0.1 dB for a network with up to 30 cascaded Bragg gratings. However, when the grating birefringence is on the order of 10⁻⁴, the induced power penalty can be as high as 5.6 dB for a network with 14 cascaded Bragg gratings.     

Performance comparison of wavelet packet transform based and conventional coherent optical OFDM transmission system

In this work, the performance of WPT-COOFDM system is investigated and compared to that of FFT-COOFDM system over a fiber link. Simulation results show that the longer length of wavelet filters achieve a better performance, whereas the complexity is higher. For different wavelet mother functions employed in WPT-COOFDM systems, the chromatic dispersion robust of John64E wavelet outperforms that of other wavelets, and which could be a viable alternative for coherent optical OFDM to be considered in short distance transmissions. The simulation results also show that most of the developed wavelet mother functions mainly for image processing are not suitable for COOFDM transmission for its sensitivity to chromatic dispersion.     

Geometrical optics analysis for reduction of trapezoidal image distortion in a single prism-based optical fingerprint scanner

A single prism-based optical fingerprint scanner structure is mathematically analyzed by using geometrical optics approach. Important parameters including a tilting angle of the fingerprint accepting plane, a geometrical-optic distant difference, a trapezoidal image distortion, an object compression ratio, and an orientation of a two-dimensional image sensor are formulated for the first time in terms of all three angles of the prism and the prism material. In addition, based on our mathematical model, we propose to design all three angles of the prism in such a way that the plane accepting the fingertip observed from one side of the prism is normal to the optical axis of the system. In this way, the imaging plane is perpendicular to the optical axis, eliminating the trapezoidal image distortion and leading to ease of implementation. Experimental verification using three commercially available dispersion prisms made from three different glass materials and one prism based on our design concept shows that a prism with higher refractive index provides lower trapezoidal image distortion. The experimental data generally obeys our theoretical analysis.     

Control of modulation and soliton polarization instabilities in photonic crystal fibers with birefringence management

Photonic crystal fibers with composite linear birefringence, that is induced by both microstructure geometry and stress applying parts, permit the management of the wavelength dependence of both group and phase birefringence. In this work we investigate the novel nonlinear propagation phenomena that are enabled by birefringence management. Vector modulation instability is enhanced near the zero phase birefringence wavelength. Soliton polarization instability is controlled by Raman self-frequency shifting across the zero phase birefringence wavelength.     

梯度折射率微球回归反射材料的研究

提出一种回归反射面材的新结构,以梯度折射率微球代替传统的超高折射率匀质玻璃微球.利用四阶Runge-Kutta方法进行光线追迹的理论计算,确定了制作新型回归反射面材的工艺参数.结果表明,按此条件制作回归反射面材可大幅度降低原先对折射率的过高要求,并显著提高回归反射的性能.     

Second- and third-harmonic generations for a nondilute suspension of coated particles with radial dielectric anisotropy

We derive expressions for the effective nonlinear susceptibility tensors for both the second harmonic generation (SHG) and induced third harmonic generation (THG) of nonlinear composite materials, in which nondilute coated particles with radial dielectric anisotropy are randomly embedded in the linear host. Two types of coated particles are considered. The first is that the core possesses a second order nonlinear susceptibility and the shell is linear and radially anisotropic, while the second is that the core is linear with radial anisotropy and the shell has a second order nonlinear susceptibility. We observe greatly enhanced SHG and THG susceptibilities at several surface plasmon resonant frequencies. For the second model, due to the coating material being metallic, there exists two fundamental resonant frequencies ω_c1 and ω_c2, whose difference ω_c2-ω_c1 is strongly dependent on the interfacial parameter and the radial dielectric anisotropy. Furthermore, in both systems, the adjustment of the dielectric anisotropy results in larger enhancement of both SHG and induced THG susceptibilities at surface plasmon resonant frequencies than the corresponding isotropic systems. Therefore, both the core-shell structure and the dielectric anisotropy play important roles in determining the nonlinear enhancement and the surface resonant frequencies.     

10 Gb/s TDM passive optical networks using four wavelengths multiplexed channels

We propose a new architecture for 10 Gb/s upstream traffic in TDM-PON using externally injection-locked Fabry-Perot laser diodes (FP-LDs) in each optical network unit (ONU). Four directly modulated 2.5 Gb/s FP-LDs were injection-locked by continuous wave (CW) carriers distributed from the optical line terminal (OLT). Hence, a total of 10 Gb/s upstream traffic can be achieved. Experimental results show negligible power penalty at a transmission of 25 km standard single mode fiber (SMF) without dispersion compensation. The performance of the injection-locked FP-LD is also studied.     

Adaptive optics retinal image registration from scale-invariant feature transform

With the use of adaptive optics(AO), high-resolution microscopic imaging of the living human retina in the single cell level has been achieved. In an AO retinal imaging system, with a small field size (about 1°, 300 μm) the motion of the eye severely affects the stabilization of the real-time video images results in significant distortions of the retina images. Scale-invariant feature transform (SIFT) algorithm is applied to automatically abstract corner points with subpixel resolution and match the points in two frames. With the matched corner points, we estimate and remove the motions of 20 frames of photoreceptor cells and capillary blood vessels, respectively. The maximal translational motion is about 30 and 44 pixels in the 20 frames whose size is 416 × 416 pixels. More general motions can be considered by the SIFT algorithm, but only simple translational motion can be considered by cross-correlation algorithm.     

Hartmann－Shack波前传感器的Fourier光学分析

Ｈａｒｔｍａｎｎ－Ｓｈａｃｋ波前传感器是自适应光学中几种重要的波前传感器之一。本文利用Ｆｏｕｒｉｅｒ光学方法分析研究了Ｈａｒｔｍａｎｎ－Ｓｈａｃｋ波前传感器的原理，导出了辐照度分布质心与子孔径波前倾斜的解析表达式，说明了辐照度分布质心及其算法的物理意义，总结了该传感器的若干重要特点。     

Filtering characteristics of film-coated long-period fiber gratings operating at the phase-matching turning point

We present novel filtering characteristics of film-coated long-period fiber gratings (LPFGs) operating at phase-matching turning point (PMTP). The 3 dB bandwidth of a single broadband dip depends greatly on the film refractive index and thickness. A π-phase-shift in the grating center produces two band-rejection peaks between which the separation reaches 375 nm. This separation increases with the number M (M > 1) of π-phase-shifts that divide the LPFG uniformly. The central loss is constant at zero for odd M, whereas decreases with M for even M. Increasing film thickness and refractive index result in blue shifts of the dual peaks and enlarge the separations, which exhibits excellent tunabilities.     

Alternating approach of implementing frequency encoded all-optical logic gates and flip-flop using semiconductor optical amplifier

In conduction of parallel logic, arithmetic and algebraic operations, optics has already proved its successful role. Since last few decades a number of established methods on optical data processing were proposed and to implement such processors different data encoding/decoding techniques have also been reported. Currently frequency encoding technique is found be a promising as well as a faithful mechanism for the conversion of all-optical processing as the frequency of light remains unaltered after refection, refraction, absorption, etc. during the transmission of light. There are already proposed some frequency encoded optical logic gates. In this communication the authors propose a new and different concept of frequency encoded optical logic gates and optical flip-flop using the non-linear function of semiconductor optical amplifier.     

The cost benefits of multi-layer architecture with multi-granularity switching in optical networks

The benefits of using multi-granularity switching to reduce the number of ports, including both electronic and optical ports, have been investigated. A novel multi-layer ring architecture with multiple switches is proposed and two general equations to count the number of ports are derived. Comparisons are made between the multi-layer multi-granularity switching (MM-XC) architecture and the reported architectures: multi-switching (M-XC) and single switching (S-XC) architectures. The results show that the M-XC or S-XC architectures can be deduced from the MM-XC architecture and the latter architecture can achieve more considerable savings of the port count compared to the former.