Low-loss three-dimensional waveguide crossings using adiabatic interlayer coupling

The silica-based planar lightwave circuit (PLC) is a promising technology for application to a reconfigurable optical add/drop multiplexer (ROADM), which contains arrayed waveguide gratings (DEMUX/MUX), variable optical attenuators (VOAs), switches (SWs), and tap monitors (TAPs) as shown in Fig. 1. Specifically, single-chip integration with higher delta (D) contrast waveguides, which allow a small bending radius, enables us to realise compact and low-cost ROADM modules [1]. The critical problem with regards to single-chip integration is the large excess loss caused by conventional two-dimensional (2D) waveguide crossings as shown by the broken line in Fig. 1, because a large channel number of more than 40 ch is strongly required in ROADMsystems, and the crossing loss will accumulate in proportion to the channel number. On the other hand, three-dimensional (3D) waveguide crossings are attractive because they are fundamentally lossless [2, 3]. The most important technical aspect with regards to 3D waveguide crossings is to find a way to interconnect between vertically stacked waveguides. Various approaches have been proposed for realising interlayer coupling. One approach is direct writing using a femtosecond laser [2]. The problem with this approach is that the writing process time increases in proportion to the number of channels. Another approach involves using vertical directional couplers to realise interlayer coupling of the double waveguide structures [3, 4]. In this case, a severe fabrication tolerance is required if we are to form a directional coupler with 100% coupling. To overcome these problems, we propose and demonstrate 3D waveguide crossings based on a triple waveguide layer structure and adiabatic double/stacked-core modefield (MF) converters. By adopting simple mode conversion with the middle waveguide layer, we can provide stable low-loss interlayer coupling between vertically stacked upper and lower waveguides.     

用飞秒激光在石英玻璃体内写入光波导和光栅的研究

用飞秒激光在石英玻璃写入了光波导和光栅等结构。飞秒激光辐射照形成波导效应时的折射率改变量为0.001至0.008。折射率的改变量依赖于飞秒激光脉冲辐照的剂量和功率密度。通过Raman光谱和AFM图像,研究了波导区的物质结构变化,并对飞秒激光写入过程的物理机制进行了探讨。     

Low-loss Na-glass waveguides

A new method for the fabrication of low-loss Na-glass waveguides on SiO2has been found. The reglassification process has enabled us to reduce the scattering loss in the planar waveguide to less than 1 dB/cm and to obtain channel waveguide with 2 dB/cm attenuation by the lift-off method or by the direct laser writing method.     

飞秒激光直写透明光学材料光波导的研究进展

综述了飞秒激光直写光波导的加工过程和表征方法、可直写形成光波导的不同透明光学材料以及直写光波导应用的进展。总结了飞秒激光直写引起的折射率变化与材料有关,同时还依赖于加工的脉冲能量、脉冲宽度、偏振以及扫描速度等。指出飞秒激光微加工在光子器件领域的有很好的应用前景。     

Spectral Loss Characterization of Femtosecond Laser Written Waveguides in Glass With Application to Demultiplexing of 1300 and 1550 nm Wavelengths

Femtosecond laser written waveguides in glass were characterized across the full telecom spectrum to gain insight into waveguide loss mechanisms, and to aid in the design of a low-loss 1300/1550 nm wavelength demultiplexer. A lambda^-4 wavelength scaling of propagation loss confirms Rayleigh scattering as a principal loss mechanism. Laser exposure was optimized for generating low-loss directional couplers with high isolation between the 1300 and 1550 nm bands. Dispersive coupling in the straight and curved wavelength regions was balanced with a 1.5-fold difference in 1300 and 1550 nm beat lengths, leading to the first demonstration of 1300/1550 nm demultiplexer written with a laser. A minimum interaction length of 3.2 mm, ~2 dB insertion loss and channel isolations of 16.7 and 18.8 dB are reported.     

A novel deposit/spin waveguide interconnection (DSWI) for semiconductor integrated optics

We propose an efficient and simple optical interconnection between active semiconductor components by deposition and spin coating. Details of the waveguide design, the fabrication technique, and a promising material combination are given. Experimental results with an integrated laser-polyimide/SiOx(x sim 2) waveguide combination demonstrate low-threshold (2.0 kA/cm²) laser operation and a low-loss waveguide interconnection (81 percent coupling efficiency) on a GaInAsP/InP chip.     

激光直写技术制备SiO2-TiO2条形光波导工艺研究

为了研究激光直写技术在光波导制备中的应用,采用波长为1.07μm的连续光纤激光器制备了硅基SiO2-TiO2条形光波导。探讨了激光直写技术制备条形光波导的原理,研究了激光参数对条形光波导宽度的影响,最后测试了光波导的通光模场以及光传输损耗。结果表明,条形光波导的宽度随着激光功率密度的增加而增大。当激光扫描速率在0.1mm/s~1mm/s范围内变化时,条形光波导的宽度随着激光扫描速率的增加而降低;高于1mm/s时对波导宽度无明显影响。在优化的工艺参数下,激光直写得到的条形波导的厚度约为0.4μm,宽度为120μm,整条波导非常均匀、准直性很好,对于1550nm波长的光呈多模传输,最小传输损耗为1.7dB/cm。     

Improvements in mode-locked semiconductor diode lasers using monolithically integrated passive waveguides made by quantum-well intermixing

By using the technique of quantum-well intermixing (QWI), monolithically integrated passive, and active waveguides can be fabricated. It is shown that mode-locked extended cavity semiconductor lasers with integrated low-loss passive waveguides display superior performance to devices in which the entire waveguide is active: the threshold current is a factor of 3-5 lower, the pulsewidth is reduced from 10.2 ps in the all active laser to 3.5 ps in the extended cavity device and there is a decrease in the free-running jitter level from 15 to 6 ps (10 kHz-10 MHz).     

Laser-fabricated low-loss single-mode waveguiding devices in GaAs

A maskless laser etching technique was used to fabricate novel waveguides and waveguiding structures directly into the surface of GaAs/AlGaAs heterostructures. The modal and loss properties of these groove-defined structures have been measured as a function of waveguide geometry, and low-loss single-mode waveguides have been produced. The technique was used to fabricate various passive optical devices in a single processing step. Waveguide bend and branch losses were measured and are comparable to those in conventionally fabricated devices. Experimental results are described by simple theoretical models. The technique is attractive as a prototyping tool for developing and testing new integrated optic circuits     

Low-Loss Y-Branch Waveguides Designed by Wavefront Matching Method

In this paper, we describe the procedure for designing low-loss Y-branch waveguides by the wavefront matching (WFM) method, and report experimental results as proof of concept. The designed Y-branches were fabricated using silica-based planar lightwave circuit (PLC) technology. A Y-branch fabricated with a 0.45%-Delta waveguide exhibited a low excess loss of less than 0.2 dB over a wide wavelength range of 1250 to 1650 nm. In addition, we demonstrate that the WFM-designed Y-branches enable us to provide compact 1 times 32 splitters with an average insertion loss of 16.0 dB at a wavelength of 1550 nm. In addition, we present some experimental results obtained using samples with different Delta values, and show that our design method is more efficient for a higher Delta waveguide suitable for functional PLC devices.     

Femtosecond Laser Application to PLC Optical Devices and Packaging

Using tightly focused femtosecond laser pulses, we produce an optical waveguide and devices in transparent materials. This technique has the potential to generate not only channel waveguides, but also three‐dimensional optical devices. In this paper, an optical splitter and U‐grooves, which are used for fiber alignment, are simultaneously fabricated in a fused silica glass using near‐IR femtosecond laser pulses. The fiber‐aligned optical splitter has a low insertion loss, less than 4 dB, including an intrinsic splitting loss of 3 dB and excess loss due to the passive alignment of a single‐mode fiber. Finally, we present an output field pattern, demonstrating that the splitting ratio of the optical splitter becomes approximately 1:1.     

Wet-etched ridge waveguides in y-cut lithium niobate

By the technique of nickel indiffusion proton exchange (NIPE) and the technique of buffered proton exchange (PE) melt, wet-etched ridge waveguides in y-cut LiNbO₃ are fabricated for the first time. The fabricated ridge waveguides have smooth surfaces and are good enough for low-loss waveguides. Moreover, a ridge waveguide Mach-Zehnder modulator in y-cut LiNbO₃ is fabricated. The measured half-wave voltage is about 30% lower than that of a conventional modulator. The wet-etched ridge waveguides in y-cut and z-cut substrates are also characterized for comparison     

Equalization in coherent lightwave systems using microwavewaveguides

The maximum bit rate/ distance product in recent single-frequency laser direct-detection lightwave system experiments has been limited by dispersion. An equalization technique, appropriate for coherent lightwave systems, that uses a microwave waveguide for overcoming the delay dispersion problem is considered. Results show that small low-loss waveguides can be used to greatly reduce dispersion. For example an 8 GHz bandwidth signal transmitted over 68 km of fiber can be equalized by a waveguide with a cross section of 6 mm×3 mm and a length of only 17 cm. With the waveguide equalizer, the dispersion-limited maximum bit rate/distance product for a standard fiber system can be increased to that of a dispersion-shifted fiber system at 1.55 μm, e.g., a 16-fold increase in maximum bit rate for 100 km transmission     

Determination of Refractive Indices From the Mode Profiles of UV-Written Channel Waveguides in ${hbox {LiNbO}}_{3}$-Crystals for Optimization of Writing Conditions

We report on a method for the simultaneous determination of refractive index profiles and mode indices from the measured near-field intensity profiles of optical waveguides. This method has been applied to UV-written single-mode optical waveguides in ${hbox {LiNbO}}_{3}$ for the optimization of the writing conditions. The results for the waveguides written with light of the wavelengths 275, 300.3, 302, and 305 nm for different writing powers and scan speeds reveal that for optimum writing conditions a maximum possible refractive index change of $sim$0.0026 can be achieved at a value of 632.8 nm transmitting wavelength. The computation process used in the presented technique may also become useful to extract absolute refractive index values of any slowly varying graded index waveguide.      

High-density integrated optics

This paper presents two dimensional (2-D) finite difference time domain (FDTD) simulations of low-loss right-angle waveguide bends, T-junctions and crossings, based on high index-contrast waveguides. Such structures are essential for the dense integration of optical components. Excellent performance characteristics are obtained by designing the waveguide intersection regions as low-Q resonant cavities with certain symmetries and small radiation loss. A simple analysis, based on coupled mode theory in time, is used to explain the operation principles and agrees qualitatively with the numerical results     

Observation of stimulated emission, line narrowing, and red shiftof emission peak from optically-pumped rectangular cross-sectional GaNoptical waveguides fabricated by selective growth

The authors examined low-loss III-V nitride optical waveguides for fabrication on sapphire substrates for active devices using a selective-area growth technique. A rectangular cross-section waveguide can be created by adjusting crystal growth conditions. A stimulated emission peak at 362 nm, line narrowing, and a red shift were observed from the selectively grown GaN waveguides by optical pumping     

Study and design of step-index channel waveguide bends withlarge-angle and low-loss characteristics

By using micro-prisms, improved three-dimensional (3-D) bends of the embedded and buried waveguides of step-index profile are proposed. A simple phase compensation rule for the optimal design of the micro-prism is also presented. Through the simulation of 3-D semivectorial finite-difference beam propagation method, the transmission characteristics of the improved bends are shown to have been enhanced dramatically as compared with those of the conventional ones. Even for a bend angle of as large as 10°, the normalized transmitted power can still be greater than 95%. These results of 3-D bends are then compared with those of the two-dimensional (2-D) ones which are simplified from 3-D structures by the effective index method, and physical explanation of the discrepancy between the 3-D and 2-D results is introduced. The influences of waveguide structures and prism parameters on the transmission characteristics are discussed in detail. Some criteria for the design of large-angle low-loss 3-D improved bends are also accessed     

Leaky Waveguide: TE/sup cir//sub 01/ Circular Waveguide Periodic Array of Circular Apertures

This paper describes propagation characteristics of periodic leaky waveguides for helix waveguides and solid metal waveguides carrying a TE/sup cir//sub 01/ mode based primarily on experimental studies. For low-loss leaky waveguides, this type has not been considered feasible, because of the critical problem encountered in mode conversion-reconversion at the circular apertures. Additional loss due to the periodic array of the circular apertures and TE/sup cir//sub 01/ loss/frequency characteristics of the periodic leaky waveguide using helix waveguides and solid metal waveguides are measured by the shuffle-pulse method in the X-band. The periodic leaky waveguides using solid metal waveguides are seen to be acceptable for practical use as a low-loss leaky waveguide system, according to these measurements.     

Neodymium-doped tantalum pentoxide waveguide lasers

The fabrication, spectroscopic properties, and laser performance of Nd/sup 3+/-doped Ta/sub 2/O/sub 5/ channel waveguide lasers are described. Lasing is obtained at both 1.066 and 1.375 /spl mu/m with threshold pump powers as low as 2.7 mW. The rib waveguides are reactive-ion-etched into Nd:Ta/sub 2/O/sub 5/ layers formed by reactive magnetron sputtering. These high-index low-loss rare-earth-doped waveguides are fabricated on silicon substrates and offer the potential for integration with photonic crystal structures for compact optical circuits.     

Pluggable Optical Board Interconnection System With Flexible Polymeric Waveguides

Pluggable optical board interconnection system using flexible optical waveguide for direct-coupling to reduce the transmission loss is proposed and demonstrated. For the direct coupling between waveguides of boards, we use optical plug and adaptor with flexible polymeric waveguide and guide pins. Flexible polymeric optical waveguide is fabricated using hot embossing technique. Eye pattern and jitter characteristics of the backplane system is presented at 10 Gb/s.