Free-space optical links for board-to-board interconnects

We describe two free-space optical links for multichannel optical interconnects. The targeted aggregate data rate is 240 Gb/s. In one implementation we use a unique implementation of telecentric optics and achieved an optical link that is simple, robust, and modular. We describe a simple, inexpensive telecentric lens that can accommodate all optical channels within a 1-mm-diameter sweet zone. We also describe the performance of an optical link with a 4×4 array of VCSELs and matching detectors. The integrity of the optical link is not significantly degraded with a>±2 mm translational misalignment between the VCSEL and detector arrays. With the telecentric optical link, we need only two low-bandwidth, single-axis active servomechanisms to compensate for static tilt and possibly low-frequency thermally-driven shift between the transmitter and receiver arrays. In the other implementaion we use two matching arrays of 1×12 optical fibers. Data are coupled optically through an air gap of 2 mm by means of a pair of collimating microlens arrays that are aligned to one another via precision mechanical subassemblies. We describe a simple, inexpensive, and robust mechanical coupling for the optical link achieved by using miniature high-flux magnets.     

Integrated receiver architectures for board-to-board free-space optical interconnects

In many computer and server communications copper cables and wires are currently being used for data transmission and interconnects. However, due to significant shortcomings, such as long transmission time, high noise level, unstable electrical properties, and high power consumption for cooling, researchers are increasingly turning their research interests toward alternatives, such as fiber optic interconnects and free-space optical communication technologies. In this paper, we present design considerations for an integrated receiver for high-speed free-space line-of-sight optical interconnects for distortion-free data transmission in an environment with mechanical vibrations and air turbulences. The receiver consists of an array of high-speed photodiodes for data communication and an array of quadrant photodiodes for real-time beam tracking in order to compensate for the beam misalignment caused by vibrations in servers. Different configurations for spatially positioning the quadrant and data photodiodes are discussed for 4×4 and 9×9 multielement optical detector arrays. We also introduce a new beam tracking device, termed the strip quadrant photodiodes, in order to accurately track highly focused optical beams with very small beam diameter.     

Low-loss coupling between two single-mode optical fibers with different mode-field diameters using a graded-index multimode optical fiber

We present a method for ultra-low-loss coupling between two single-mode optical fibers with different mode-field diameters using multimode interference in a graded-index multimode optical fiber. We perform a detailed analysis of the interference effects and show that the graded-index fiber can also be used as a beam expander or condenser. The results are important for devices in which optical fibers with different mode-field diameters are coupled in series, such as in ultra-short-pulse fiber ring lasers, or in optical fiber communication links.     

Advanced multifunctional optical switch for multimode optical fiber networks

In this work, an advanced multifunctional optical switch based on multimode fibers is proposed. It can work as a 3 × 1 optical multiplexer/combiner, a 2 × 2 optical switch, a variable optical attenuator and a variable optical power splitter. All these functionalities can be developed in the same device without any hardware modification, only by using the proper ports and control electronics.The proposed switch has been developed for being used in the visible and near infrared wavelength range: 450–650 nm for optical fiber automobile applications, 650–850 nm for home broadband applications; and 850–1300 nm for multimode fiber access networks. Up to three different types of twisted nematic liquid crystal cells have been designed and fabricated for fulfilling these different wavelength ranges as part of the proposed device.The multifunctional switch has been implemented and experimentally tested. Crosstalk usually better than ? 15 dB at 532 nm, 660 nm and 850 nm, in any state has been measured. Switching is achieved at voltage levels of 4 Vrms. Fiber to fiber insertion losses when operating as a 2 × 2 optical switch, range from 10 to 15 dB within 200 nm wavelength range; with a non-optimized optics for collimation and coupling.     

Bending-induced optical loss in random-hole optical fibers

We present the experimentally determined optical bend loss for random hole optical fibers in the spectral range 1520 to 1570 nm induced by wrapping the optical fiber around a fixed diameter mandrel. The optical losses are compared to those obtained for a single-mode fiber and a multimode fiber using the same bending procedures. The bending induced optical losses in the random hole optical fibers were several orders of magnitude lower than for the single-mode fiber and were about 1 order of magnitude lower than for the multimode fiber.     

Optical tapping element for multimode fibers

We report on an optical tapping structure for multimode fibers. Its characteristic features are: 1. simple fabrication; 2. good reproducibility; 3. simple technology; 4. high coupling efficiency. The principle of the tap is based on a lateral displacement of waveguides which are butt joined. The displacement defines the fraction of power to be tapped off. With a lateral displacement of 15μm and a fiber with 100 μm outer diameter and 90 μm core diameter (acceptance angle ±30°) we measured a total coupling efficiency of 74% (73% remains in the main trunk line, ? 1% was tapped off) even though this fiber according to its high acceptance angle was not suited for the geometry of our structure. A diagram is given which shows the coupling efficiencies as a function of the lateral displacement of the fibers.     

All propagation modes of large-core multimode optical fibers with an arbitrary core profile

Plastic optical fibers are typical large-core multimode optical fibers. High-bandwidth graded-index plastic optical fibers, which support a great number of propagation modes compared with conventional silica-glass multimode optical fibers, were developed in the 1990s. However, because they support a great number of propagation modes, their modal analyses have been limited to the WKB analysis. We obtain all the propagation modes of a convex-index large-core multimode optical fiber by use of the finite-element method, which has a strong advantage for arbitrary core profiles.     

Nanosecond laser-induced surface damage of optical multimode fibers and their preforms

High-power optical multimode fibers are essential components for materials processing and surgery and can limit the performance of expensive systems due to breakdown at the end faces. The aim of this paper is the determination of laser-induced damage thresholds (LIDT) of fibers (FiberTech) and preforms (Heraeus Suprasil F300). Preforms served as models. They were heated up to maximum temperatures of 1100, 1300 and 1500°C and cooled down to room temperature at rates of 10 K min⁻¹ (oven) and ∼10⁵ K min⁻¹ (quenched in air) to freeze in various structural states simulating different conditions similar to a drawing process during the production of fibers. Single- and multi-pulse LIDT measurements were done in accordance with the relevant ISO standards. Nd:YAG laser pulses with durations of 15 ns (1064 nm wavelength) and 8.5 ns (532 nm) at a repetition rate of 10 Hz were used. For the preforms, LIDT values (1-on-1) ranged from 220 to 350 J/cm² (1064 nm) and from 80 to 110 J/cm² (532 nm), respectively. A multi-pulse impact changed the thresholds to lower values. The LIDT (1064 nm wavelength) of the preforms can be regarded as a lower limit for those of the fibers.     

Double-clad fiber for endoscopy

Endoscopes employing a single optical fiber may have advantages over conventional fiber-bundle or CCD array imaging techniques, including the potential for greater flexibility and miniaturization. Although single-mode fibers can provide superior resolution compared with multimode fibers, they are prone to increased speckle noise and suffer from limited optical throughput and reduced depth of field. We demonstrate the use of a double-clad fiber for single-mode illumination and multimode detection to achieve high-resolution, reduced-speckle imaging with high optical throughput and a large depth of field.     

N × N集成光开关阵列模型

报道了由2N个1 × N 多模干涉马赫-曾德尔光开关组成的N × N光开关阵列结构,分析了这种结构的开关阵列优势和局限性.用场传输矩阵方法建立了1 × N多模干涉光开关的光场传输方程.给出了光开关阵列从任一输入端输入、从任一输出端输出时阵列开关的工作条件.在上述原理及理论基础上分析了4×4光开关阵列的结构和工作条件.     

A Surface Mounting Embedded Optical Transceiver with Bi-Directional Data Rate of Eight Channels × 10 Gbps

Abstract

The bi-directional data rate is investigated of an eight-channel × 10-Gbps optical transceiver with a size of 15 mm × 15 mm × 6 mm and ball grid array package form that was designed and fabricated. A passive optical coupling method is designed based on a coupling lens array, which is transformed to substrate through a carrier. This electrical performance is characterized through a 3D full wave simulation and shows great advantages compared with traditional pluggable optical transceivers. Experimental eye diagram measurement in loop-back mode via a 2-m-long multi-mode fiber array shows an eye width of 51.984 ps at a bit-error rate order of 10^?12, which proves the transceiver's ability for 10.3125-Gbps data transmission.     

All polarization-maintaining Er:fiber-based optical frequency comb for frequency comparison of optical clocks

We demonstrate an optical frequency comb (OFC) based on a turnkey mode-locked laser with a figure-9-shape structure and polarization-maintaining fibers, for the comparison of frequency among optical clocks with wavelengths of 698 nm, 729 nm, 1068 nm, and 1156 nm. We adopt a multi-branch approach in order to produce high power OFC signals at these specific wavelengths, enabling the signal-to-noise ratio of the beatnotes between the OFC and the clock lasers to exceed 30 dB at a resolution bandwidth of 300 kHz. This approach makes the supercontinuum spectra much easier to be generated than a single branch OFC. However, more out-of-loop fibers degrade the long-term frequency instability due to thermal drift. To minimize the thermal drift effect, we set the fiber lengths of different branches to be similar, and we stabilize the temperature as well. The out-of-loop frequency instability of the OFC due to the incoherence of the multi-branch is about 5.5×10^-19 for 4000 s, while the in-loop frequency instability of f_ceo and that of f_beat are 7.5×10^-18 for 1 s and 8.5×10^-18 for 1 s, respectively. The turnkey OFC meets the requirement for the comparison of frequency between the best optical clocks.     

Femtosecond fiber laser based methane optical clock

An optical clock based on an Er³⁺ fiber femtosecond laser and a two-mode He–Ne/CH₄ optical frequency standard (λ=3.39 μm) is realized. Difference-frequency generation is used to down convert the 1.5-μm frequency comb of the Er³⁺ femtosecond laser to the 3.4-μm range. The generated infrared comb overlaps with the He–Ne/CH₄ laser wavelength and does not depend on the carrier–envelope offset frequency of the 1.5-μm comb. In this way a direct phase-coherent connection between the optical frequency of the He–Ne/CH₄ standard and the radio frequency pulse repetition rate of the fiber laser is established. The stability of the optical clock is measured against a commercial hydrogen maser. The measured relative instability is 1×10⁻¹² at 1 s and for averaging times less than 50 s it is determined by the microwave standard, while for longer times a drift of the He–Ne/CH₄ optical standard is dominant.     

用金属小球进行长波量子阱红外探测器的光耦合

针对实验中9.5μm峰值响应波长的n型长波量子阱红外探测器设计运用二维金属小球(铜)阵列作光耦合结构.金属小球阵列均匀填充在绝缘的胶黏剂中，基于惠更斯原理研究二维金属小球阵列体系的光耦合和光吸收，结果表明对9.5μm响应波长的长波量子阱红外探测器，采用周期为3μm，半径为0.9μm左右的金属小球阵列可以获得最佳的光耦合.优化设计后的量子效率(66%)远高于45°磨角耦合的量子效率(38%)，为实验运用金属小球阵列进行长波量子阱红外探测器的光耦合提供了基本的理论依据和详细的优化设计方案.     

Expansion dynamics of an array of high density Bose-Einstein condensates produced in a 1D CO<Subscript>2</Subscript> laser optical lattice

We discuss the expansion dynamics under mean-field repulsion of an array of ⁸⁷Rb Bose-Einstein condensates produced in an all-optical scheme involving 1D lattice with nearly 10⁵ atoms, after fast evaporative cooling of just about 1 s. Single site occupation exceeds 2 × 10⁴ in our experiments. The possibility of transition to two-dimensionality was also investigated. The expansion behavior of the high density multiple micro-condensates produced directly in the CO₂ laser 1D optical lattice, with a lattice spacing of 5.3 μm, agrees well with a numerical simulation based on the mean-field theory.     

Fiber optic bundle array wide field-of-view optical receiver for free space optical communications

We propose a design for a free space optical communications (FSOC) receiver terminal that offers an improved field of view (FOV) in comparison to conventional FSOC receivers. The design utilizes a microlens to couple the incident optical signal into an individual fiber in a bundle routed to remote optical detectors. Each fiber in the bundle collects power from a solid angle of space; utilizing multiple fibers enhances the total FOV of the receiver over typical single-fiber designs. The microlens-to-fiber-bundle design is scalable and modular and can be replicated in an array to increase aperture size. The microlens is moved laterally with a piezoelectric transducer to optimize power coupling into a given fiber core in the bundle as the source appears to move due to relative motion between the transmitter and receiver. The optimum position of the lens array is determined via a feedback loop whose input is derived from a position sensing detector behind another lens. Light coupled into like fibers in each array cell is optically combined (in fiber) before illuminating discrete detectors.     

复合抛物面型集光器与多模光纤的耦合

分析讨论了复合抛物面型集光器与多模光纤的耦合效率和集光器设计之间的关系，提出了采用截底法和截顶法提高集光器与多模光纤耦合效率的方案。     

Indoor coverage improvement of MB-OFDM UWB signals with radio over POF system

A radio over fiber system using the fluorinated based polymer multimode fibers (PMMF) is presented in this paper for the enhancement of the indoor coverage of the multiband orthogonal frequency division multiplexing ultra-wideband standard (MB-OFDM UWB) inside a building. A preliminary part related the cost analysis owing to glass and polymer multimode fiber deployment inside a fiber network is reported. The study of the physical properties of the polymer optical fibers (core diameter, numerical aperture, differential mode delays, modal bandwidth…) is firstly performed in order to effectively exhibit the potentialities and the robustness of such fibers to be used in a low cost radio over fiber system. The DMD measurements of four fluorinated based polymer optical fiber are reported. The designed system operates at 850 nm with commercial off the shelf (COTS) devices combined to the intensity modulation/direct detection technique. The opportunity of using polymer fibers and COTS components to improve the indoor coverage of the MB-OFDM UWB standard is so reported by the measurement of the Error Vector Magnitude or the Relative Constellation Error variation as a function of the system parameters (RF power, optical attenuation, fiber length…) as well as the compliance of the eye diagram with the mask testing. By the way, the transmission performance of both 200 and 480 Mbps signals is demonstrated over up to 200 m link length of polymer multimode fibers: transmission penalties are quantified by relative constellation error with values under the standard requirements. A comparative study with classical OM2 50 μm based glass multimode fiber having the same bandwidth/length product than the PMMF is done.     

基于光纤的光学频率传递研究

随着光钟研究的发展, 光钟的稳定度和不确定度均达到10^-18量级. 通过光纤可以实现光钟频率信号的高精度传输, 有望用于未来“秒”定义的复现. 演示了百公里级实验室光纤上的光学频率传递. 对于在实验室70 km光纤盘上实现的光频传递, 光纤相位噪声抑制在1-250 Hz傅里叶频率范围内均接近于光纤延时极限, 对应传输稳定度(Allan偏差)为秒级稳定度1.2×10^-15, 10000 s稳定度为1.4×10^-18. 实验室100 km光纤的光频传递秒级稳定度也达到了5×10^-15. 提出了光纤噪声用户端补偿的方案, 可以简化星形传递网络中心站的复杂度. 在25 km光纤上演示了该传递方案, 实现的传输稳定度接近传统前置补偿传递方案.     

Varactor-tuned gunn diode voltage controlled oscillator antenna array

A simple varactor tuned X-band Gunn diode VCO antenna array which is strongly coupled has been demonstrated. These arrays have the advantages of simple biasing circuit, no resistors required to eliminate multimode problem and suitable for monolithic integration circuit. Preliminary results show a maximum tuning range of 47MHz for 1×1 array and 170MHz for 2×2 array. In order to solve power combining heating problem, we move the backside metal forward and it becomes a microstrip form. The measured frequency and radiation patterns of these grid arrays agree very well with theoretical calculations.