Optimization of Optical Add-Drop Multiplexers Based on Arrayed Waveguide Gratings with Fabrication Errors for Transparent Optical Networks

A transparent optical network (TON) composed by optical add-drop multiplexers based on arrayed waveguide grating (AWG) with fabrication errors is optimized for 10 Gbit/s per channel and channel spacing of 25 GHz. The optimization takes into account the effect of inter-symbol interference, amplified spontaneous emission noise accumulation, and coherent and incoherent homodyne crosstalk. Numerical results reveal that AWG fabrication errors can have a high influence on the TON performance; however, in all investigated situations, they do not affect the AWG parameters values corresponding to the optimum AWG frequency response. This surprising behavior is due to the fabrication errors affect mainly the AWG response outside the passband for the required AWG crosstalk level. It is shown that the optimum AWG frequency response is a flat-top response with a mean amplitude response outside the passband lower than -30 dB and the switch isolation should exceed 30 dB.     

Evaluation of Transparent Optical Network Performance in Presence of Incoherent Homodyne Crosstalk

Abstract

A simple method to evaluate the performance of transparent optical networks (TON) that takes into account the incoherent homodyne crosstalk, signal distortion, inter-symbol interference (ISI), amplified spontaneous emission (ASE) noise, and arbitrary optical and electrical filters is proposed. This method combines the effect of the incoherent homodyne crosstalk with an exhaustive Gaussian approximation for the influence of ASE noise, signal distortion, and ISI. It is shown that the incoherent homodyne crosstalk penalty depends on the ASE noise accumulated in the TON, ISI, and extinction ratio. The proposed method estimates are compared with experimental results published by other authors. Good agreement is observed for one, two, and four incoherent homodyne crosstalk signals, revealing that the proposed method provides accurate estimates even when the methods based on a Gaussian approximation for the incoherent homodyne crosstalk fail. The proposed method is used to assess the performance of a TON composed by optical add-drop multiplexers based on arrayed waveguide gratings. Results reveal that incoherent homodyne crosstalk, ASE noise, and ISI impair the TON performance and, therefore, none can be neglected in the TON design.     

Design and characterization of arrayed waveguide gratings using ultra-low loss Si3N4 waveguides

Planar waveguides with ultra-low propagation loss are necessary for integrating optoelectronic systems that require long optical time delay or narrowband optical filters. In this paper, we review an ultra-low loss planar waveguide platform that uses thin (<150 nm) Si₃N₄ cores and thick (>8 μm) SiO₂ cladding layers. In particular, we discuss the performance of arrayed waveguide gratings (AWGs) fabricated with the platform. We propose the use of a practical design method that takes the statistical nature of worst-case crosstalk into account. We also demonstrate the measurement of amplitude and phase error distributions in an AWG using an optical backscatter reflectometer. We show that the waveguides have phase errors small enough to achieve AWG crosstalk below ?30 dB, while crosstalk below ?40 dB should also be possible with optimization of the component design.     

Calculation of the spectral response of an arrayed-waveguide gating demultiplexer with a wide-angle beam propagation method in a cylindrical coordinate system

The spectral response of an arrayed-waveguide grating (AWG) demultiplexer is calculated by simulating the field propagation in the output section of an AWG with a wide-angle beam propagation method (BPM) in a cylindrical coordinate system. As in a practical design of an AWG demultiplexer, each output waveguide consists of two straight sections connected by a bending section. The spectral response obtained by the present algorithm is more accurate than those obtained with two popular approximate methods, namely, the conventional overlapped integral method and the standard BPM for radially straight and infinitely long output waveguides. With the present algorithm, the dependence of the spectral response on the parameters of the output section is analyzed. The channel crosstalk and the 3 dB passband width of the spectral response depend mainly on the length of the first straight section, the end separation and the angular separation of the output waveguides. The bending section results in an asymmetrical spectral response with remarkable sidelobes which can be reduced by increasing the bending radius.     

一种有效提高薄膜滤光片型密集波分复用器器件性能的方法及其实验研究

对光梳状滤波器加薄膜滤光片型模块制作密集波分复用器(DWDM)的两种应用方案进行了研究,提出了一种提高插损一致性、信道隔离度及减小串扰的结构方案。对新结构方案与常规结构方案进行了理论分析及实验研究,结果表明新结构可将级联次级峰由大于-30dB降至-50dB以下。用16波50GHz的密集波分复用器件拼接进行的实验表明最终器件的插损减小0．869dB,插损一致性减小2．005dB,相邻信道隔离度提高1．004dB,非相邻信道隔离度提高42．903dB．总串扰提高1．68dB。该方案不仅可以应用到光梳状滤波器与薄膜滤光片型模块拼接高性能的超密集波分复用器件．同样也可适用于阵列波导光栅等类型的密集波分复用器件中以降低工艺难度,提高性能指标。     

Transmission passband,optical crosstalk and cascadability of free-space concave grating demultiplexer for dense WDM networks

We describe the performance of a 100 channel free-space concave grating demultiplexer with 1dB transmission passband of ±7GHz (FWHM 14–22GHz). The influence of optical aberrations and defocusing on the transmission spectrum is demonstrated. Crosstalk between neighbouring channels separated by >0.3nm is lower than –25dB and no penalty due to crosstalk is observed using 2.6Gbit/s directly modulated DFB lasers. Penalty-free transmission through a cascade of 30 demultiplexers is demonstrated in an optical fibre recirculating loop experiment, and the permitted tolerances on the laser frequency misalignments for a larger number of cascaded demultiplexers are investigated.     

Tunable Optical Add/Drop Multiplexers Using Cascaded Mach Zehnder Coupler

Abstract

A tunable add/drop filter based on Cascaded Mach Zehnder (CMZ) coupler using SiO₂/silicon oxinitride (SiON) is presented with its mathematical model. The increase of filtered width (i.e., free spectral response) and lowering of crosstalk for this filter are optimized using its mathematical model with truncated binomial coupler distribution. The number of wavelength channels with 100 GHz (0.8 nm) channel spacing for 5-stage (M = 9, r = 2) truncated binomial CMZ filter with index contrast ～5% at ?20 dB crosstalk and the bending loss of 0.1 dB per MZ section are obtained as ～37 (free spectral range of 31 nm). It is seen that if during the fabrication process, waveguide core width w is increased or decreased by 0.1 μm (in percentage terms ～±6.6%), the crosstalk is slightly increased by ～7%. Thermal tuning for wavelength channels is achieved by thin film heater with low power thermooptic delay line structure which reduces the heating power by ～1.58 times in comparison to the conventional structure.     

Acousto-Optic Tunable Filter (AOTF) Revisited: Ultrashort Optical Pulses Crosstalk Studies on the Lossy Filter

We examine again the crosstalk level (Xtalk) and extinction ratio (Xratio) of an acousto-optic tunable filter (AOTF) operating with ultrashort light pulses (2 ps), now considering the loss effect. The compression factor was studied for the switched pulse with lossy AOTF (α _dB = 4 dB/mm) considering five nonlinearity profiles. One can observe that there is always an optimum value for δ (final value of the nonlinearity) that one can obtain a switched pulse with the same time duration of the input pulse (C = 1). The compression factor for the switched pulse along the filter length shows that for the constant profile, the compression factor is around 1 all over the device. The study of the crosstalk level (Xtalk) of this device, considering the optimum values of δ obtained from the compression studies, as a function of the pump power (P ₀) was done. The acousto-optic tunable filter (AOTF) has attracted great attention in recent years, in part because it appears to be a suitable basis for multi-wavelength optical cross-connects. It is probably the only known tunable filter that is capable of selecting several wavelengths simultaneously. This capability can be used to construct a multi-wavelength router.     

Evaluation of Transparent Optical Network Performance in Presence of Incoherent Homodyne Crosstalk

A simple method to evaluate the performance of transparent optical networks (TON) that takes into account the incoherent homodyne crosstalk, signal distortion, inter-symbol interference (ISI), amplified spontaneous emission (ASE) noise, and arbitrary optical and electrical filters is proposed. This method combines the effect of the incoherent homodyne crosstalk with an exhaustive Gaussian approximation for the influence of ASE noise, signal distortion, and ISI. It is shown that the incoherent homodyne crosstalk penalty depends on the ASE noise accumulated in the TON, ISI, and extinction ratio. The proposed method estimates are compared with experimental results published by other authors. Good agreement is observed for one, two, and four incoherent homodyne crosstalk signals, revealing that the proposed method provides accurate estimates even when the methods based on a Gaussian approximation for the incoherent homodyne crosstalk fail. The proposed method is used to assess the performance of a TON composed by optical add-drop multiplexers based on arrayed waveguide gratings. Results reveal that incoherent homodyne crosstalk, ASE noise, and ISI impair the TON performance and, therefore, none can be neglected in the TON design.     

Design and tolerance analysis of optical interleaver based on retardant crystals

A set of recursion equations was developed for the design of an optical interleaver based on retardant crystals and a third-order optical interleaver with maximally flat passband was designed accordingly. The fabrication tolerance for the retardant crystals was simulated based on crosstalk consideration. Given crosstalk requirement of −25 dB, the tolerance for the crystal thickness and axis orientation was obtained as 0.91 μm and 0.51°, respectively.     

Design and Simulation of Broadband Beam Splitter on a Silicon Nitride Platform for Optical Coherence Tomography

ABSTRACT

Integrated photonics enables the miniaturization of bulk optical components for biosensing applications such as optical coherence tomography (OCT) and is therefore promising for future lab-on-chip solutions. Here, we report the design and simulation of a compact low loss broadband beam splitter with arbitrary coupling ratios on silicon nitride platform for OCT systems. The reported coupler uses asymmetric waveguide-based phase control section for 10:90, 20:80, 30:70, 40:60, and 50:50 splitting ratios and is broadband over 100 nm with the central wavelength of 850 nm. The couplers are realized for transverse electric, transverse magnetic, and fully vectorial modes, and maximum excess loss for all mode types is reported to be less than 0.19 dB. The design tolerance of waveguide width and thickness of the designed coupler is further calculated and is within fabrication limit.     

Analysis for fabrication errors of arrayed waveguide grating multiplexers

Based on the transmission theory, parameter optimization is performed, and effects of fabrication errors on transmission characteristics are analyzed for a 33×33 polymer arrayed waveguide grating (AWG) multiplexer around the central wavelength of 1550.918 nm with the wavelength spacing of 0.8 nm. Simulated results show that fabrication errors result in the shift of the transmission spectrum, and lead to the increase of the crosstalk compared with the device theoretically designed. Furthermore, accumulation and compensation of fabrication errors are investigated. In order to realize the normal demultiplexing of the fabricated AWG device, the allowed fabrication errors are discussed.     

Dispersion resulting from phase and amplitude errors in arrayed-waveguide grating multiplexers-demultiplexers

We report dispersion resulting from phase and amplitude errors in arrayed-waveguide grating (AWG) multiplexers-demultiplexers. We measure the phase- and amplitude-error distributions of two different types of AWG and discuss their effects on the dispersion. The results show that the origin of the dispersion in the AWG is the low-order Fourier components of the symmetric phase and the antisymmetric amplitude errors. We also discuss the dependence of the dispersion on the passband shape in the presence of the same fabrication errors.     

A concise design of 16 × 16 polymer AWG with low insertion loss and crosstalk

In this paper, a 16-channel arrayed waveguide grating multiplexer (AWG) has been designed using polymer materials with 1.5% refractive index difference. Certain important parameters are optimized using the coupling mode theory and Beam Propagation Method. The factors that affect the insertion loss and the crosstalk are analyzed in this paper. In our design we introduced the parabolic taper structure and evaluated the suitable number of the arrayed waveguide, obtaining a total insertion loss of 2.19 dB. For obtaining a low crosstalk we evaluate the pitches of adjacent input/output waveguides ΔX and arrayed waveguides d as different values. We chose the value of ΔX about 2.5 times of d by enlarging the pitches of adjacent input/output waveguides. The crosstalk of the designed AWG is lower than −40 dB.     

一种基于斜光栅辅助的非对称耦合器型光分插复用器

提出了一种基于斜光栅辅助的非对称耦合器型光分插复用器。运用复合波导的三维正交模式,对器件的三种可能的结构进行了理论分析,选出粗波导光栅型结构。利用耦合模理论,模拟了斜光栅的耦合特性并对其倾斜角进行优化设计。通过回波峰值设计法,将器件的工作波长放在波分复用信道之内,回波峰值波长放在波分复用信道之外,使得器件的性能大有提高。模拟结果表明器件的串扰可达到-30 dB,回波损耗可达到-25 dB。同时,器件的关键工艺容差较大,易于批量化生产。当斜光栅的倾斜角度在2.5°到4.5°之间时,器件的串扰低于-28 dB,回波损耗低于-22 dB。     

Performance Evaluation and Characterization of Hybrid Optical Amplifiers for Dwdm Systems at Ultra Narrow Channel Spacing

We study hybrid optical amplifiers (HOAs) for dense wavelength division multiplexed (DWDM) systems at reduced channel spacing (<0.2 nm). The Raman erbium-doped fiber amplifier (EDFA) (R-E) and EDFA-semiconductor optical amplifier (SOA) (E-S) hybrid optical amplifiers (HOAs) are investigated for the first time at 0.05 and 0.1 nm of the channel spacing. We show that the R-E HOA provides better gain (23.9 dB) and induces lesser crosstalk (-14.1 dB) with minimum utilization of the bandwidth. We report that 70 km is the optimum span distance at which the R-E HOA achieves a 2450 km transmission distance with acceptable performance. The theoretical results are found to be in accordance with numerical simulations in terms of the amplifier gain and induced crosstalk.     

一种提高基于循环频移器的多载波光源光信噪比的方案

频率锁定多载波光源是实现太比特系统传输的关键器件之一, 目前已成为光纤通信领域的研究热点.基于循环频移技术产生的频率锁定 多载波光源因具有驱动电压低、串扰小、平坦度高的优点而受到重视, 但是其输出质量容易受到调制器固有的高阶谐波串扰的影响,特别是三阶谐波串扰. 为了减小三阶串扰的影响,本文提出了在I/Q调制器两支路上再加载另一频率为3f_m的射频信号的方案, 用其产生的一阶信号来抑制三阶串扰.通过理论分析与数值仿真,得到了载波数为24、 载波间最大功率差小于0.1 dB的多载波光源输出.与未采用三阶串扰抑制方案的结果比较, 光源输出有效光信噪比提高了2 dB. 结果表明,采用本文的改进方案,可以有效地提高多载波光源的输出平坦度和有效光信噪比.     

Compact SOI arrayed waveguide grating demultiplexer with broad spectral response

A compact eight-channel flat spectral response arrayed waveguide grating (AWG) multiplexer based on silicon-on-insulator (SOI) materials has been fabricated on the planar lightwave circuit (PLC). The 1-dB bandwidth of 48 GHz and 3-dB bandwidth of 69 GHz are obtained for the 100 GHz channel spacing. Not only non-adjacent crosstalk but also adjacent crosstalk are less than −25 dB. The on-chip propagation loss range is from 3.5 to 3.9 dB, and the total device size is 1.5 × 1.0 cm².     

基于Harvey-Shack表面散射理论的光学误差测量

基于散射理论,不同频段的光学表面制造误差会对光学性能产生不同的影响,而常用的光学设计软件一般没有考虑。为此利用小波变换对误差进行了频段分割;然后基于Harvey-Shack表面散射理论,从频段误差的角度对光学表面的光学性能进行了评价,同时基于小波变换的特点,当光学性能不满足要求时,找到了需重点控制的频段误差在光学表面发生的区域,从而对下一步的加工进行指导。最后以一块口径500 mm的大镜实测数据及设计要求"在0.33 mrad内环绕能量大于70%"进行了实验验证。结果表明,利用此方法能有效的建立"表面频段误差光学评价光学加工"三者之间的联系。     

行波电极偏振无关干涉型光开关

提出并实现了一种Ｘ切ＬｉＮｂＯ３上的行波电极偏振无关干涉型光开关。达到的指标为：串间地－１４ｄＢ，偏振无关的开关电压为１９Ｖ，带宽达到６ＧＨｚ以上。进一步减小定向耦合器部分中光波导间距的分级步距，串音可达到－２０ｄＢ以下 。